Class 9 Chemistry - Atoms And Molecules

Define ion.

$\text{Ion :-}$ It is an electrically charged atom or molecule formed by loss or gain of electrons.

Ion formed by loss of electron is called cation. Example:- Magnesium ion $\longrightarrow Mg^{2+}$
Ion formed by gain of electron is called anion. Example:- Cyanide ion $\longrightarrow CN^-$

Define Symbol.

Symbol:-

A chemical symbol is a representation of chemical elements using one or two letters.
Every element of the periodic table is represented by a different symbol. Example:- Sodium $\longrightarrow Na$

Why does $OH^{ - }$ is an ion?

Any atom or group of atoms that bears one or more positive or negative electrical charges is known as an ion. A positively charged ion is called a cation and a negatively charged ion is called an anion.

$OH^-$ is an ion because it contains a negative charge and is known as hydroxide ion or anion.

According to Dalton,______is the smallest indivisible particle of the matter.

According to Dalton’s atomic theory, all matter, i.e., an element, a compound or a mixture is composed of small particles called atoms. Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.

Complete the statements given below by filling in the blank with the correct word/s.
The basic unit of an element is an ________.

The basic unit of an element is an

$\underline{atom}$ .

Fill in the blanks:
The molecules are made up of _________.

The molecules are made up of

$\underline{\textbf{atoms}}$ .

When two or more atoms are chemically bonded together they form a molecule. Molecules can be made up of same kind of atoms or different kinds of atoms.

__________ are made up of same kind of atoms.

In chemistry, an

$element$ is a pure substance that cannot be broken down by chemical means, consisting of identical atoms.

Answer=

$element$

An ion with positive charge is called

$\text{cation}$ .

Complete the statements given below by filling in the blank with the correct word/s.
Atom contains , with positively charged _.

Atom contains

$\underline{nucleus}$ , with positively charged

$\underline{protons}$ .

Select the correct answer from the choice in brackets.
The symbol of — Mercury [Mg / Hg / Ag]

The symbol of Mercury -

$Hg$ .

If an atom loses an electron, it acquires a ____ charge.

$positive$
Atom is an electrically neutral entity that consists of an equal number of protons and electrons. So if it loses an electron, it contains one proton in excess w.r.t the electrons. Hence if an atom loses an electron, it acquires a net positive charge.

If an atom gains an electron, it is called _____.

If an atom gains an electron, it is called

$\underline {anion}$ .

For example: Chlorine atom gains an electron to become chloride anion.

$Cl+e^- \to Cl^-$

State Dalton's atomic theory.

Dalton's atomic theory states that:
1. Matter is made up of tiny particles called atoms.
2. Atoms are indivisible.
3. Atom can be neither be created nor destroyed.
4. Atoms same elements are identical in mass and properties.
5. Atoms of different elements differ in mass and properties.

A charged particle is called an _____.

A charged particle is called an ion.
Any atom or group of atoms that bears one or more positive charge is called cation and one or more negative charge is called anion.

An ion with negative charge is called as_________.

An ion is an atom or group of atoms that has lost one or more electrons is known as the cation(positively charged ion), or that has gained one or more electrons is known as an anion(negatively charged ion).

What does the word 'atom' mean?

The word atom is derived from the Greek word 'atomos', which means a thing that can't be split. It is the smallest unit into which matter can be divided.

What are molecules ?

Molecules are formed when two or more atoms of the same or different elements combine. Molecules have an independent existence. Molecules are the smallest particle of a compound. Examples:

$H_2O$ molecule,

$O_2$ molecule.

Name the smallest particle from which matter is made up.

The smallest particle from which matter is made up is

$atom$ .

Write the names of elements from the following symbol :
Be

Every chemical symbol starts with a capital letter, with the second letter written in lower case. For example, Be - Beryllium

Give some more examples of molecules in the respective categories (other than the ones already mentioned).

1800069_1850188_ans_e7592d9e949340ab88bb25b1d60c17de.png

Write the diatomic molecules from the following
$H_2, Na, C, N_2, P_4, O_2$

Diatomic molecules are those molecules which contain two atoms. Hence,

$H_2, N_2, O_2$ are diatomic molecules among the given molecules.

How many moles of hydrogen are present in 1 gm of hydrogen?

Molar mass of

$H=1 g/mole$ .

No. of moles=

$\dfrac{given\ mass}{molar\ mass}$

No. of moles in

$1g$ of

$H=\cfrac{1g}{1g/mole}=1mole$ .
Thus

$1$ mole of

$H$ is present in

$1$ g of

$H$ .

Calculate the molecular mass of ethanoic acid , $CH_3COOH$ .
(Atomic masses : $C = 12u ; H = 1u ; O = 16u$ )

Molecular mass of ethanoic acid

$(CH_3COOH) = 1 \times C + 3 \times H + 1 \times C + 2 \times O + 1 \times H = 12 + 3 + 12 + 32 + 1 = 60u$

Give the symbols for metals of group $13$ .

The metals of the group 13 of the periodic table and their symbols are Aluminium(Al), Gallium(Ga), Indium(In) and Thallium(Tl).

The weight of $1 \times 10^{22}$ molecules of $\mathrm{CuSO}_{4} \cdot 5 \mathrm{H}_{2} \mathrm{O}$ is _________.

Molecular mass of

$CuSO_4.5H_2O = 249.5$ gm

$= \dfrac{249 \cdot 5}{6 \cdot 02 \times 10^{23}} \times 10^{22}=4 \cdot 14 g$

Components in a mixture are present in varying proportions and not in a fixed proportion.

The fact that a mixture does not have a fixed proportion only differentiates it from compounds. If the proportion becomes fixed and the elements get bonded then it will become a compound. e.g. Sugar dissolved in water is a mixture and its proportion is not fixed i.e. Someone may make a more concentrated solution and someone a less concentrated one.

Molecular mass of water is __________ u.

Molecular formula of water is

$H_2O$ . Atomic mass of

$H = 1 \ u$ and atomic mass of

$O = 16\ u$ . Thus, molecular mass of

$H_2O= 2\times 1 + 16 = 18\ u$ .

Molar mass of $C_2H_6$ (in g/mol) is:
(Given atomic weight of $C = 12$ gm and $H = 1$ gm)

Molar mass of

$C_2H_6 = 12\times2+ 1 \times 6 = 30 \ g/mol.$

Calculate the molar mass of $\displaystyle { CO }_{ 2 }$ .

Molar mas of

$\displaystyle CO_2 = \text {atomic mass of carbon} + 2 \times \text {atomic mass of oxygen}$

$\displaystyle \text {Molar mass of } CO_2 = 12 \: g/mol + 2(16 \: g/mol) = 44 \: g/mol$

Give reason why an atom of an element is electrically neutral?

An atom of an element is electrically neutral because the number of positive protons is equal to the number of negative electrons.

Calculate the molar mass of $\displaystyle { CH }_{ 4 }$ .

$\displaystyle \text {Molar mass of methane}\ (CH_4) = \text {Atomic mass of carbon} + (4 \times \text {Atomic mass of hydrogen})$

$\displaystyle \text {Molar mass of methane} = 12 + (4 \times 1) = 16 \: g/mol$

Define atom.

Atom is the smallest particle of an element which may or may not be capable of independent existence. It can take part in chemical reactions to form the products.

Determine the molecular mass of water.

Hydrogen has an atomic mass of

$1$ and oxygen atomic mass of

$16$ , so

$H_2O$ has a mass of

$16 + 2 \times 1=18 \ g/mol$ .

What is meant by a 'mole' of a substance?

The mole is the unit of measurement in the International System of Units (SI) for the amount of substance. It is defined as the amount of a chemical substance that contains as many elementary entities (e.g., atoms, molecules, ions, electrons, or photons) as the Avogadro constant( $N_A$ ), which has a value of $6.022×10^{23}mol^{−1}$ .

Give an example of the following:

A molecule in which central atom is linked to three other atoms.

A molecule in which the central atom is linked to three other atoms-

$Cl-\underset {|}{Al}-Cl$

$Cl$

Aluminium Chloride

It is a lewis acid since its valency is

$3$ and it can accept a pair of electrons to complete its octet.

How many moles are there in $200g$ of $Na$ ?

Molecular mass of

$Na=23g/mole$

$No. \ of \ mole=\cfrac{mass}{molar\ mass}$

$200g$ of

$Na$ moles is:

$n=\cfrac{200g}{23g/mole}$

$n=8.69 \ moles \ of \ Na$

Define the following terms:
Atom
Isotope
Atomic number
Atomic mass number

(1) Atom: The smallest part into which an element can be divided.

(2) Isotope: Species having same atomic number but different mass number.

(3) Atomic number: Number of electrons present in an atom which is equal to number of protons in a neutral atom.

(4) Atomic mass number: Sum of the number of protons and neutrons i.e, nucleons present in an atom.

Write a short note on Avogadro's number .

Avogadro's number is the number of units in one mole of any substance, equal to

$6.0221408\times 10^{23}$ . The unit may be electrons, atoms,ions or molecules depending upon the nature of the substance and the character of the reaction.

It is the number of atoms in exactly 12 grams of C-12. It is a dimensionless quantity.

Which qualities are required in the student to perform such an activity?

Team management and basic concepts are required to perform such an activity.

What is the difference between cation and anion?

Cations have a net positive charge while anion has a net negative charge. Cations and anions are both ions but The difference between a cation and an anion is the net electrical charge of the ion. Ions are atoms or molecules which have gained or lost one or more valence electron giving the ion a net positive or negative charge. Cations are ions with a net positive charge. ex.

$Na^+$ ion and Anions are ions with a net negative charge. ex.

$Cl^-$ ion

Calculate the number of moles of magnesium present in a magnesium ribbon weighing 12 g. molar atomic mass of magnesium is $24 g mole^{-1}$

1288278_1300075_ans_56ef65e1583447099bc39928b34b3f48.jpg

How do negative and positive ions form?

When an atom gains or loses an electron, it forms negative or positive ions respectively.

A negatively charged ion is called an ‘anion’ and a positively charged ion, a ‘cation’.

What is an atom?

Explanation:

An atom is the smallest unit of an element.
In an atom, subatomic particles like protons and neutrons are present inside the nucleus and negatively charged electrons are present in the extranuclear part.

Define an atom.

Hint: Atoms are the building blocks of matter.

Part 1: Atom

An atom is the smallest constituent unit of matter that possesses the properties of a chemical element.

Part 2: Structure of an atom

Atoms are composed of three elementary particles: protons, electrons, and neutrons.
The atom's nucleus contains protons (positively charged particles), and neutrons (neutral).

The atom's outermost region contains electrons (negatively charged particles).

Part 3: Examples of the atom

Helium (He) atom, Neon (Ne) atom, Hydrogen (H) atm, Carbon (C) atom, Nitrogen (N) atom, etc.

Calculate the number of hydrogen molecules in $100 \,gm$ hydrogen.

We have to find the number of molecules of hydrogen in 100 gram hydrogen.

The molecular mass of hydrogen is

$2\times1 = 2u$

1 mole of hydrogen molecule

$= 2\ g$

This means

$2\ g$ of hydrogen molecules contains

$= 6.022\times 10^{23}$ molecules

Therefore,

$100\ gm$ of hydrogen molecules contains

$=\dfrac{6.022\times 10^{23}}{2}\times 100$

$= 3.011\times 10^{25}$ molecules

Calculate the mass (in grams) of -
(i) $10^{23}$ molecules of $CO_{2}$ $gas$
(ii) 2 moles of nitrogen atom
(C=12 u, 0= 16 u, N = 14 u)

$\textbf{(i)}$

$\textbf{Given data:}$

Number of molecules of $CO_2$ gas = $10^{23}$

$\textbf{Step 1: Calculate the molar mass of carbon dioxide gas.}$

Molar mass of carbon dioxide ( $CO_2$ ) = Mass of C atom + Mass of 2 O atoms.

Molar mass of carbon dioxide ( $CO_2$ ) = 12 + (2 \times 16)

Molar mass of carbon dioxide ( $CO_2$ ) = 12 + 32 = 44 g/mol

$\textbf{Step 2: Calculate the number of moles of carbon dioxide gas.}$

$Number\ of\ moles\ of\ carbon\ dioxide = \dfrac{number\ of\ molecules\ of\ carbon\ dioxide}{Avogadro's\ number}$

$Number\ of\ moles\ of\ carbon\ dioxide = \dfrac{10^{23}}{6.022 \times 10^{23}}$

$Number\ of\ moles\ of\ carbon\ dioxide = 0.166 mol$

$\textbf{Step 3: Calculate the mass of carbon dioxide gas.}$

$Number\ of\ moles\ of\ carbon\ dioxide = \dfrac{Mass\ of\ carbon\ dioxide}{Molar\ mass\ of\ carbon\ dioxide}$

$0.166 = \dfrac{Mass\ of\ carbon\ dioxide}{44}$

$Mass\ of\ carbon\ dioxide = 0.166 \times 44 = 7.304 g$

$\textbf{Conclusion:}$

Therefore, the mass of $10^{23}$ molecules of $CO_2$ gas is 7.304 g.

$\textbf{(ii)}$

$\textbf{Given data:}$

Number of moles of nitrogen atom = 2 moles

$\textbf{Step 1: Calculate the mass of nitrogen atom.}$

$Number\ of\ moles\ of\ nitrogen\ atom = \dfrac{Mass\ of\ nitrogen\ atom}{Atomic\ mass\ of\ nitrogen}$

$2 = \dfrac{Mass\ of\ nitrogen\ atom}{14}$

$Mass\ of\ nitrogen\ atoms = 2 \times 14 = 28 g$

Conclusion:

Therefore, the mass of 2 moles of nitrogen atom is 28 g.

Fill in the blank.
Dalton said that _______ could not be divided.

Dalton said that atoms could not be divided. All matter is composed of extremely small particles called atoms. Atoms cannot be subdivided, created, or destroyed.

$AB_2$ and $A_2B_3$ are two compounds of the elements A and B. 0.15 mole of each of these compounds weighs 9.3 and 15.9 g respectively. Find the atomic masses of A and B.

Let molar mass of A be

$x \ g\ mole^{-1}$

Let molar mass of B be

$y \ g\ mole^{-1}$

$Number \ of \ moles= \dfrac{Given \ mass}{Molar \ mass}$

0.15=

$\dfrac{9.3}{x+2y}$ and 0.15 =

$\dfrac{15.9}{2x+3y}$

$x+2y=62$ and

$2x +3y=106$

$y=18 \ g\ mole^{-1}$ and

$x=26\ g\ mole^{-1}$ .

Hence the atomic mass of x is

$26 \ u$ and y is

$18 \ u$

A sample of $CaCO_3$ has Ca (40%), C (12%), O (48%) by mass. If the law of constant proportions is true then find the mass of Ca in 5 g of $CaCO_3$ obtained from another source.

Law of constant proportions states that a given chemical molecule will always contain the same elements in same proportions, irrespective of the source.

Since, it is given that a sample of

$CaCO_3$ has

$40$ %

$Ca$ in it.

$\therefore$ The percentage of

$Ca$ in

$5\ g$ of sample would be the same as in

$100\ g$ of

$CaCO_3$

$\therefore$

$5\ g$ sample

$CaCO_3$ will have

$40$ %

$Ca$ i.e;

$\cfrac {40}{100}\times 5 = 2\ g$

Calculate the number of atoms in $120\ g\ of\ Ca$ .

$\text{Step 1: Finding the number of moles in 120g of Calcium(Ca)}$

$Number\ of\ moles=\dfrac{Given\ Mass}{Molar\ mass}$

$Number\ of\ moles=\dfrac{120}{40}$

$Number\ of\ moles=3 mol$

$\text{Step 2: Calculate the number of atoms in 120g of Calcium}$

Since,

$1 mole= 6.023\times 10^{23}\ atoms$

$Thus, the\ number\ of\ atoms\ in\ 3\ moles\ of\ Calcium= 3\times 6.023\times 10^{23} atoms$

$= 18.069\times 10^{23} atoms$

Write formula of - barium sulphate

The chemical formula of Barium sulphate is

$BaSO_4$

The step-by-step method for writing the chemical formula of Barium sulphate is as follows:

$\textbf{Step 1: Write the symbols of the elements/groups}$

On the left side	On the right side
Barium $Ba$	Sulphate $SO_4$

$\textbf{Step 2: Write the valencies of the ions}$

The valency of Ba is 2 (Barium being a metal loses 2 electrons and forms

$Ba^{2+}$ ion). And the valency of sulphate ion is 2.

On the left side	On the right side
Positive ion $Ba^{2+}$	Negative ion $(SO_4)^{2-}$

$\textbf{Step 3: Interchange the valencies of ion}$

The valencies of both the ions are interchanged by using the criss-cross valency method.

The valencies are now reduced to the lowest possible number and therefore the chemical formula of barium sulphate is

$BaSO_4$ .

$\text{Ravi prepared a solution of sodium chloride by mixing 5.85 g salt in 1 litre of water. Find}$
Molar mass of sodium chloride

$\textbf{Given data:}$

Given mass of sodium chloride = 5.85 g

Volume = 1 L

$\textbf{Step 1: Calculate the molar mass of sodium chloride.}$

Molar mass of sodium chloride ( $NaCl$ ) = Mass of Na atom + Mass of Cl atom

Molar mass of sodium chloride ( $NaCl$ ) = 23 + 35.5 = 58.5 g/mol

$\textbf{Conclusion:}$

Therefore, the molar mass of sodium chloride is 58.5 g/mol.

The mass of one steel screw is 4.11 g. Find the mass of one mole of the steel screws. Compare this value with the mass of the Earth $(5.98 \times10^{24})$ kg.Which one of the two is heavier and by how many times?

Mass of one steel screw= 4.11 g

Thus, mass of one mole of screw

$=4.11\times N_A=4.11\times 6.022\times 10^{23}$

One mole of screws weigh

$2.475\times 10^{24}g$ =

$2.475\times 10^{21 }kg$

$\dfrac{Mass \ of \ the\ earth}{Mass \ of \ 1 \ mole \ of \ screws}$ =

$\dfrac{5.98 \times10^{24}kg}{2.75\times10^{21}kg} = 2.410^3$

Mass of earth is

$2.410^3$ times the mass of screw

The earth is 2400 times heavier than a mole of screw.

14 g of $N_2$ contains $3.01\times 10^{23}$ nitrogen molecules. If true then enter 1, if false enter 0.

Answer: True (1)

Mass of one mole

$N_2$ =

$1 \times molar\ mass$ =

$1 \times (2 \times 14)$ =

$28$ g

Number of molecules in one mole of

$N_2$ or

$28$ g

$N_2$ = Avogadro Number of molecules =

$6.023 \times 10^{23}$ molecules

Number of molecules in

$14$ g

$N_2$ =

$\dfrac {6.023 \times 10^{23}}{2}$ =

$3.01 \times 10^{23}$ molecules

The molar mass of $H_2O$ is:

Molar mass of

$H_2O = 2 \times 1+16 = 18\:g/mol$ .

The molar mass of $CO_2$ is :

Molar mass of

$CO_2 = 12\times1 +16 \times 2 = 44\ g/mol$ .

Molar mass of $H_2SO_4$ is:
(Given: Atomic mass of $H=1, S=32, O=16$ )

Molar mass of

$H_2SO_4 = 2\times 1+32+16\times 4 = 98$ g/mol.

If the atomic mass of carbon were to set at $100$ amu, the Avogrado's number will be equal to $5.01\times {10}^x$ atoms. Find the value of $x$ .

As we know, the Avogadro number is the number of atoms in one mole of carbon.
So, if we change mass of carbon to

$100$ amu then, Avodrado number is

$6.022\times10^{23}\times100/12 = 5.01\times {10}^{24}$ .
So, value of

$x$ is

$24$ .

Select the correct answer from the given choices for the statement given below.
A positively charged ion is called [cation / anion]________.

A positively charged ion is called

$\mathrm{cation}$

An atom or molecule when loses an electron becomes a positively charged species called a cation. The species accepting this lost electron from cation becomes a negative species called an anion.

Cations are formed by .............. (loss/ gain) of electrons and anions are formed by ................. (loss/ gain) of electrons.

Cations are formed by loss of electrons and anions are formed by gain of electrons.

Example-

$X- 2e^- \rightarrow X^{++}$

$Y + 3e^- \rightarrow Y^{3-}$

Identify the odd one out:
Mercury, Gold, Brass, Nitrogen

Mercury, gold and nitrogen are pure elements while brass is an alloy of zinc and copper. So, brass is odd one out of the four.

The weight of $1\times 10^{22}$ molecules of $CuSO_4\cdot 5H_2O$ is:
(nearest integer)

Weight of

$6.023\times 10^{23}$ molecules

$=$ Molecular weight of

$CuSO_4\cdot 5H_2O=249 g$

$=$

$1$ mol of

$CuSO_4\cdot 5H_2O$

Weight of

$1\times 10^{22}$ molecules of

$CuSO_4\cdot 5H_2O$

$=\dfrac {249\times 1\times 10^{22}}{6.023\times 10^{23}}=4.13 g$

$\approx$

$4$

What is denoted by the chemical formula $S_8$ .

The chemical symbol of sulphur is

$S$ .

$8$ here denotes eight atoms of sulphur.
Hence

$S_8$ denotes that a molecule of sulphur that consists of eight sulphur atoms.

What does the symbol $H_2$ convey?

$H_2$ : Two atoms of hydrogen combined chemically to form a molecule of hydrogen.

Calculate the formula mass of compound given below:
NaCl (common salt)
Given: Atomic masses of Na = 23, Cl = 35.5.

Formula mass or formula unit mass of a substance is the sum of the atomic masses of all atoms in a formula unit. Formula mass of

$NaCl = 23 + 35.5 = 58.5\ u$

Calculate the molar mass of the following:

(i) $\displaystyle { H }_{ 2 }O$

(ii) $\displaystyle { CO }_{ 2 }$

(iii) $\displaystyle CH_{ 4 }$

(i) Molar mass of water

$(H_2O)$

Atomic mass of

$H = 1\ u$

Atomic mass of

$O = 16\ u$

$Molar\ mass\ of\ water\ (H_2O) \displaystyle = (2 \times atomic\ mass\ of\ H) + (1\times atomic\ mass\ of\ O)$

$= (2 \times 1) + (1 \times 16)= 18 \: g/mol$

(ii) Molar mass of carbon dioxide

$\displaystyle (CO_2)$

Atomic mass of

$C = 12\ u$

Atomic mass of

$O =16\ u$

$Molar\ mass\ of\ (CO_2) = (1\times atomic\ mass\ of\ C) + (2 \times atomic\ mass\ of\ O)$

$= (1 \times 12) + (2\times 16) = 44 \: g/mol$

(iii) Molar mass of methane

$(CH_4)$

Atomic mass of

$C = 12\ u$

Atomic mass of

$H= 1\ u$

$Molar\ mass\ of\ methane\ (CH_4) = (1\times atomic\ mass\ of\ C)+ (4\times atomic\ mass\ of\ H)$

$= (1 \times 12) + (4\times 1) = 16\ g/mol$

What will be the mass of one $\displaystyle ^{ 12 }{ C }$ atom in g ?

$^{12}C$ is a stable isotope of carbon. The atomic number of

$^{12}C$ is 6.

$\text{Molar mass (mass of one mole) is 12 and $$1 mole = 6.022}\times 10^{23} atoms$

$\therefore 6.022\times 10^{23} \text{atoms have mass} = 12 g$

$\therefore \text{1 atom have mass} = \dfrac{12}{6.022\times 10^{23}} = 1.99 \times 10^{-23}g$

$\textbf{Conclusion}$

mass of

$^{12}C$ is

$1.99\times10^{-23}g$

Calculate the mass and charge of one mole of electrons.

Given:
Mass of one electron = $9.10 \times 10^{-31}$ kg
Charge of one electron = $1.602 \times 10^{-19}$ coulomb

Mass of 1 mole of electrons:

$\displaystyle 9.10 \times 10^{-31} \times 6.023 \times 10^{23}= 5.486 \times 10^{-7}\ kg$

Charge of 1 mole of electrons:

$\displaystyle 1.602 \times 10^{-19} \times 6.023\times 10^{23} = 9.647\times 10^{4}\ coulomb$

What are the difference between atoms and molecules?

Atom may be defined as the smallest particle of an element that does not exist free in nature but takes part directly in chemical combinations.

Atom of any element is represented by the symbol of that element. For example, hydrogen atom is represented by

$H,$ sodium atom is represented by

$Na,$ etc.

Molecule may be defined as the smallest particle of an element or compound which exists free in nature but does not participate directly in chemical combinations.

Molecule of any substance is represented by their molecular formula. For example, water molecule is represented by

$H_2O$ . It tells that each water molecule contains two atoms of hydrogen and one atom of oxygen.

In an experiment 0.2430 gm of magnesium on burning with oxygen yielded 0.4030 gm of magnesium oxide. In another experiment 0.1820 gm of magnesium on burning with oxygen yielded 0.3020 gm of magnesium oxide. Show that the data explain the law of definite proportions.

Experiment 1: Weight of Magnesium oxide

$= 0.4030$ gm

Weight of Magnesium

$= 0.2430$ gm

Weight of oxygen

$= 0.4030 – 0.2430 = 0.16$ gm

Ratio of Magnesium: oxygen

$= 0.2430: 0.16 = 1.552: 1$

Experiment 2:

Weight of Magnesium oxide

$= 0.3020$

Weight of Magnesium

$= 0.1820$

Weight of oxygen

$= 0.3020 – 0.1820 = 0.12$

Ratio of magnesium: oxygen

$= 0.1820: 0.12 = 1.552: 1$

In both experiments the ratio of magnesium: oxygen is same (

$1.518:1$ )

Hence, it illustrates the law of definite proportions.

Mention the difference between $N_2$ and $2N$ ?

$N_2$ represents one molecule of nitrogen which consists of two atoms and

$2N$ represents two separate atoms of nitrogen.

Convert into mole.
$(a)$ $12$ g of oxygen gas
$(b)$ $20$ g of water
$(c)$ $22$ g of carbon dioxide.

(a) Molecular weight of oxygen in grams = 32 g of oxygen= one mole of oxygen gas

Thus, 12 g of oxygen

$=\frac { 1 }{ 32 } \times 12=0.375\ moles$

(b) Molecular weight of water

$(H_2O)$ in grams= 2+ 16=18 g=one mole of water

Thus, 20 g of water

$=\frac { 1 }{ 18 } \times 20=1.111\ moles$

$(CO_2)$ in grams

$= 12+2\times 16=44\ g=$ one mole of carbon dioxide

Thus, 22 g of carbon dioxide

$= \frac {1}{44}\times 22=0.5\ moles$

The symbol for element lead is derived from the latin name ---------------.

Lead's chemical symbol comes from the Latin word for waterworks, plumbum.

A symbol of an element is always represented by the first alphabet of an atom of the element.

$\text{False.}$

For example, sodium which is the first group element is represented by

$\text{Na}$ and not by its first letter. Hence it is not necessary that all elements are represented by their first alphabet.

Match the mass of elements given in column I with the no. of moles given in column II and mark the appropriate choice.

A. Atomic mass of

$He=4 g$

Number of moles

$=\dfrac{mass}{atomic\ mass}$

$n=\dfrac{28}{4}=7$ moles

B. Atomic mass of

$Na=23 g$

Number of moles

$=\dfrac{mass}{atomic\ mass}$

$n=\dfrac{46}{23}=2$ moles

C.Atomic mass of

$Ca=40 g$

Number of moles

$=\dfrac{mass}{atomic\ mass}$

$n=\dfrac{60}{40}=1.5$ moles

D.Atomic mass of

$Al=27 g$

Number of moles

$=\dfrac{mass}{atomic\ mass}$

$n=\dfrac{27}{27}=1$ mole

The sodium salt of methyl orange has 7% sodium. What is the minimum molecular weight of the compound?

Sodium has molar mass

$M$

$_{sodium}=23$ g/mol.

Methyl orange has 7% sodium

$\therefore M \times \dfrac{7}{100} = 23$

$\therefore M=\dfrac{23}{0.07}=328.57 \,g/mol$ .

$Answer:\, 328.57 \,g/mol$ .

Which of the following contains the largest number of oxygen atoms? 1.0 g of $O$ atoms, 1.0 g of $O_2$ , 1.0 g of ozone $O_3$ ?

Data:-

Weight of

$O=1.0g$

Weight of

$O_2=1.0g$

Weight of

$O_3=1.0g$

Formula:-

(1) Number of moles=

$\cfrac {given \ mass }{molar \ mass}$

(2) Number of atoms=Number of moles

$\times$ Avogadro's number

(i) For

$O$ , Number of moles=

$\cfrac {1.0}{16}=0.0625$ [molar mass of

$O=16 \ g\ mole^{-1}$ ]

Number of atoms

$= 0.0625\times 6.022\times 10^{23}$

$=0.376 \times 10^{23}$

(ii) For

$O_2$ , Number of moles=

$\cfrac {1.0}{32}=0.031$ [molar mass of

$O_2=32 \ g \ mole^{-1}$ ]

Number of atoms

$= 2\times 0.031\times 6.022\times 10^{23}$

$=0.376 \times 10^{23}$

(iii) For

$O_3$ , Number of moles=

$\cfrac {1.0}{48}=0.020$ [molar mass of

$O_3=48\ g \ mole^{-1}$ ]

Number of atoms

$= 3\times 0.020\times 6.022\times 10^{23}$

$=0.376 \times 10^{23}$

Answer:- All three will contain the same number of atoms.

The order of increasing relative molecular mass of the following gases is, hydrogen, oxygen, carbondioxide, sulphurdioxide, chlorine. Given 8 gram of each gas at STP, which will contain the least number of molecules and which the most?

Molecular masses of given molecule:-

$H_2<O_2<CO_2<SO_2<Cl_2$

$2<32<44.01<64<71$

Number of moles=

$\cfrac {Weight}{Molecular\quad mass}$ [All gases are

$8$ gram]

(i) For

$H_2$ :- Number of moles=

$\cfrac {8}{2}=4$

Number of molecules= Number of moles

$\times$ Avogadro number

$=4\times 6.022\times 10^{23}$

Number of molecules=

$24.088\times 10^{23}$

(ii) For

$O_2$ :- Number of moles=

$\cfrac {8}{32}=0.25$

Number of molecules=

$0.25\times 6.022\times 10^{23}$

$=1.505\times 10^{23}$

(iii) For

$Cl_2$ :- Number of moles=

$\cfrac {8}{71}=0.11$

$\therefore$ Number of molecules=

$0.11\times 6.022\times 10^{23}$

$=0.6624\times 10^{23}$

It can be seen that Hydrogen gas has most number of molecules and chlorine gas least number of molecules.

In two different experiments, the copper oxide was obtained from copper. The ratio of the mass of copper and the mass of oxygen found in the two experiments are the same. They are in the ratio of $4: 1$ . What of you infer about the experiments?

$2$ different experiments, copper oxide was obtained from copper and the ratio of mass of

$Cu$ and mass of

$O$ found in

$2$ experiments are same, i.e

$4:1$

This shows that the experiments are in accordance with the law of constant or definite proportions. According to this law, a chemical compound always contains exactly the same proportion of elements by mass.

Find the molecular mass of sulphuric acid.

The molar mass of hydrogen is 1

Molar mass of sulphur (S) is 32

Molar mass of oxygen (O) is 16

Now ,

2(1)+32+4(16) = 98

Molar mass of sulphuric acid i.e. $H_2SO_4$ is 98.

Find out the molecular mass of sulphuric acid.

Atomic mass of hydrogen is

$1 u$

Atomic mass of sulphur is

$32 u$

Atomic mass of oxygen is

$16u$

Molecular mass of sulphuric acid

$({ H }_{ 2 }S{ O }_{ 4 })$

$=(1 \times 2)+32+(4 \times 16)=98 u$

Give two reasons to prove that water is a compound and not an element.

Two important reasons to prove that water is a compound and not an element are :

1. Water's melting point and boiling point varies according to external pressure, volume and temperature. This implies that it is a compound and not an element as for the latter, all of it's properties remain the same under any condition.

2. Water is stable at normal conditions. This is unlike elements which tend to be reactive.

A molecule formed by combination or association of three molecules of the same substance is known as _______ .

A molecule formed by combination or association of three molecules of the same substance is known as

$trimer$ .

Aluminium reacts with sulphuric acid to form Aluminium sulphate. Answer the following:
What is the charge carried by sulphate ion?

Alumunium reacts with sulphuric acid to form Aluminium sulphate.

$Al + H_2SO_4 \longrightarrow Al_2(SO_4)_3$

Here, the charge carried by aluminium is three as

$Al^{3+}$ and the charge carried by sulphate ion is two as

$SO_4^{2-}$ . Charge carried by sulpate ion is

$-2$ .

Formation of ions from neutral atoms or molecules known as :

The process of gaining or losing electrons from a neutral atom or molecule, forming ions, is called ionization.

Calculate the total number of electrons present in one mole of methane.

1 molecule of Methane contains, 6 Carbon

$\ e^-$ and 4 Hydrogen

$\ e^ -$ , in total

$10\space e^-$

$1$ mole of Methane

$=6.023\times 10^{23}$ methane molecules

$6.023\times 10^{23}$ methane molecules contain

$=10\times 6.023\times 10^{23}$

$(1 \ methane \ molecule=10\ e^-)$

$=6.023\times 10^{24}$ electrons

Match the List of scientists and philosophers(List 1) with the list of statements(List 2).

-Maharishi Kanad was an Indian philosopher who suggested that Padarth(Matter) is made up of small particles called 'Parmanu'.Parmanu is a Sanskrit word.
-Pakudha Katyayama suggested that Parmanu combines in different ways to form different types of matter.
-Democritus and Leucippus were Greek philosophers who suggested that matter is made up of small particles called atoms. Atom is a Greek word.

(a) State the postulates of Dalton's atomic theory.
(b) Calculate the concentration in terms of mol/kg when 6.35g of KCl is dissolved in 300g of water. [At mass of K=39u, At mass of Cl=35.5u]

The

$5$ postulates are as follows:-

1) The matter consists of indivisible particles called atoms

2) Atoms are the smallest unit that takes part in chemical reactions

3) Atoms can neither be created nor destroyed

4) Atoms of the same element are similar, but that of different elements differ

5) Atoms combine in the ratio of small whole numbers to form compounds.

Molar mass of KCl

$=35 \times 35.5$

$=74.54$

$\therefore$ moles

$= \dfrac{6.35}{74.5}\times 10$

$\approx0.085$ mol

$\therefore$ concentration in mole/ kg.

$=\dfrac{0.085}{300}=\dfrac{8.5}{3}$

$\approx 2.841$ mol/kg

How many water molecules will be there in $3\times 10^{-23}$ g sample of water?

Molar mess of

${H_2}O = 18g$

no of measles

$= \frac{{3 \times {{10}^{23}}}}{{18}} \times 6 \times {10^{23\,\,\,}} = {10^{26}}$

Find the molecular mass (in kilogram) of an ammonia molecule $NH_3$ .

The atomic masses of N and H are 14 g/mol and 1 g/mol respectively. The molecular weight of ammonia is

$\displaystyle 14+3(1)=17 \ g/mol$ .

Convert unit from g to kg

$\displaystyle 17 \ g/mol \times \dfrac {1 \ kg}{1000 \ g}= 0.017 \ kg/mol$

1 mole of ammonia weighs 0.017 kg and contains Avogadro's number (

$\displaystyle 6.023 \times 10^{23}$ ) of molecules.

Molecular mass of an ammonia molecule

$\displaystyle =\dfrac { 0.017 \ kg /mol}{6.023 \ \times 10^{23} \ molecules/mol}=2.82 \times 10^{-26} \ kg$

Write the molecular mass of
$HgCO_3,CaCO_3,NaHCO_3,CH_3CH_2OH, Al_2,(SO_4)_2$

$HgCO_3 \longrightarrow (200.5)+(12)+3(16)= 260.6u$

$CaCO_3 \longrightarrow 40+12+3(16)=100u$

$NaHCO_3 \longrightarrow 23+1+12+3(16)=84u$

$CH_3CH_2OH \longrightarrow 2(12)+ 6(1)+ 16= 46u$

$Al_2(SO_4)_3 \longrightarrow 2(27)+3[32+4(16)]=342u$

where

$u$ = atomic mass units

Number of molecules present in $4.2$ g $N_2$ is $X\times10^{22}$ . What is the value of $X$ (round off to 1 digit)?

$28\,g$ of

$N_{2} = 6.02\times 10^{23}$ molecules

$4.2\,g$ of

$N_{2} = \dfrac{6.02 \times 10^{23}}{28}\times 4.2$ molecules

$\Rightarrow 9.03\times 10^{22}$ molecules

Hence, the value of

$X = 9$ .

Calculate the number of molecules in a drop of water weighing $0.05$ gm.

The molecular mass of water =

$18\ g/mol$

The number of moles of water

$=n=\dfrac{weight}{Molecular\ wt.}=\dfrac{0.05}{18}=2.78\times 10^{-3}$

Now, the number of molecules of water

$=n\times 6.022\times 10^{23}$

$=2.78\times 10^{-3}\times 6.022\times 10^{23}$

$=1.67\times 10^{21}$ molecules

In an experiment 3g of hydrogen was obtained. If hydrogen and oxygen combine in the ratio of 1:Prove the law of constant proportion.

$1\ g$ of hydrogen gas

$=8\ g$ of oxygen gas

So,

$3\ g$ of hydrogen gas requires

$=8\times 3\ g$ of oxygen gas

$=24\ g$ of oxygen gas. It is governed by the law of constant proportions.The Law of constant proportion states that a chemical compound always contains exactly the same proportion of elements by mass.

What is a mole?

A mole is defined as the amount of a substance that exactly contains $6.022 \times 10^{23}$ entities like atoms, molecules etc. Mole is the SI unit for the amount of a substance and its symbol is mol.

Cost of sugar ( ${C}_{12}{H}_{22}{O}_{11}$ ) is Rs. $40$ per kg. Calculate its cost per mole.

Let's first find the molar mass of sugar by adding up the atomic masses of all atoms in it.

Molar mass of sugar (

$C_{12}H_{22}O_{11}$ ) = (12

$\times$ 12) + (22

$\times$ 1) + (11

$\times$ 16) = 144 + 22 + 176 = 342 g/mol

Now, let's find out how many moles of sugar are in 1 kg, i.e. 1000 g sugar.

Number of moles =

$\dfrac {Mass}{Molar\ mass}$ = 1000/342 = 2.92 mol

It's given that 1 kg sugar costs Rs. 40

$\therefore$ Cost of 2.92 moles sugar = Rs. 40

Cost of one mole of sugar = 40/2.92 = 13.69 Rs. per mole

What does $^{12}C$ mean in amu?

An atomic mass unit (symbolized AMU or a.m.u) is defined as precisely 1/12 the mass of an atom of carbon-12. The carbon-12

$(^{12}C)$ atom has six protons and six neutrons in its nucleus. In imprecise terms, one AMU is the average of the proton rest mass and the neutron rest mass.

A compound contains 1 carbon atom and 2 oxygen atoms. The molecular mass of compound (in gm) is __________.

$1$ carbon and

$2$ oxygen atoms=

$CO_2$

Molecular mass

$=$

$(1 \times C)+(2 \times O)$

Molecular mass

$=$

$(1 \times 12)+(2 \times 16)$

$= 12+32$

$=44$ g

In an experiment, 1.288 g of copper oxide was obtained from 1.03 g of copper. In another experiment, 3.672 g of copper oxide gave, on reduction, 2.938 g of copper. Show that these figures verify the law of constant proportions.

(a) In the first experiment:

Weight of copper oxide,

$\left( CuO \right) = 1.288\ g$

Weight of copper

$= 1.03\ g$

Weight of oxygen

$=$ Wt. of copper oxide

$-$ Wt. of copper

$= 1.288 - 1.03 = 0.258\ g$

$\therefore \; \cfrac{\text{Wt. of copper}}{\text{Wt. of oxygen}} = \cfrac{1.03}{0.258} \approx \cfrac{4}{1}$

(b) In the second experiment:

Weight of copper oxide

$\left( CuO \right) = 3.672\ g$

Weight of copper

$= 2.938\ g$

Weight of oxygen

$=$ Wt. of copper oxide

$-$ Wt. of copper

$= 3.672 - 2.938 = 0.734\ g$

$\therefore \; \cfrac{\text{Wt. of copper}}{\text{Wt. of oxygen}} = \cfrac{2.938}{0.734} \approx \cfrac{4}{1}$

The proportion of weights of copper and oxygen in both experiments is same.

Hence, law of constant proportion is proved.

Name of the ion with symbol $S^{2-}$ _______

Name of the ion is

$\text {Sulphide ion}$ .

Give the answers of the following:
(a) What is the mass of one atom of S?
(b) What is the atomic mass of S?
(c) what is the gram atomic mass of S?

(a) The mass of one atom of

$S$ =

$\cfrac {Atomic\ mass}{6.022\times 10^{ 23 }}$

$=\cfrac {32\ amu}{6.022\times 10^{ 23 }}$

$=\ 5.31\times 10^{ -23 }\ amu$

(b) Atomic mass of

$S = 32\ amu$

$1$ mole of

$S$ in grams =

$32g$

What are anions?

An atom, that gains one or more electrons, acquires a net negative charge and is known as an anion. They are called anions as they are attracted to the anode during the electrolysis of salt solutions.

Write the complete symbol for the atoms with given atomic and mass no, $Z=18\ A=40\$ and $A=9$
( $Z=$ Atomic no and $A=$ mass)

A normal Argon atom has 18 electrons, 18 protons, and 22 neutrons. A normal atom is an atom that has a neutral charge. Since electrons are negative, protons are positive, and neutrons are neutral, that means the amount of electrons and protons is the same in a normal atom, in which case it is. In an ion, the atom will have either less or more electrons than protons, making it either positively or negatively charged.
Argon has 8 valence electrons (that means 8 of its electrons are orbiting the nucleus at the outermost orbital) making it a noble gas (the noble gases are on the rightmost part of the periodic table; Argon is the third from above).

Write the limitations of Dalton's atomic theory.

$\text{Limitations of Dalton's atomic theory -}$

It failed to explain how atoms of different elements differ from each other i.e. It did not tell anything about the structure of the atom.
It could not explain how and why atoms different elements combine with each other to form compound atoms or molecules.
It failed to explain the nature of forces that bind together different atoms in a molecule.
It failed to explain Gay Lussac's law of combining volumes.
It did not make any distinction between ultimate particle of an element that takes part in reactions (atoms) and an ultimate particle that has independent existence (molecule).

Calculate the molecular masses of $H_2, O_2, Cl_2, CO_2, CH_4, C_2H_6, C_3H_6, NH_3, CH_3OH$

Moleculer mass of

$H_2$

$=2\times Atomic$

$mass$

$of$

$H$

$=2\times 1u=2u$

Moleculer mass of

$O_2$

$=2\times Atomic$

$mass$

$of$

$O$

$=2\times 16u=32u$

Moleculer mass of

$Cl_2$

$=2\times Atomic$

$mass$

$of$

$Cl$

$=2\times 35.5u=71u$

Moleculer mass of

$CO_2$

$= Atomic$

$mass$

$of$

$C+Atomic$

$mass$

$of$

$O$

$=(12+ 32)u=44u$

Similarly,

Molecular mass of

$CH_4\\12+(4\times 1)=12+4=16u$

Molecular mass of

$C_2H_6\\24+6=30u$

Molecular mass of

$NH_3\\14+3=17u$

Molecular mass of

$CH_3OH\\12+3+16+1=32u$

$2.8\ g$ of calcium oxide prepared by heating limestone were found to contain $0.8\ g$ of oxygen. When one gram of oxygen was treated with calcium, $3.5\ g$ of calcium oxide was obtained. Show that the results illustrate the law of definite proportions.

$\underset {m}{2.8 \ gm}$ of

$CaO\longrightarrow \underset {n}{0.8 \ gm}$ of Oxygen

$\underset {x}{1.0\ gm}$ of

$O_2\longrightarrow \underset {y}{3.5\ gm}$ of

$Ca$

According to the law of definite proportions

$\Rightarrow \cfrac {m}{n}=\cfrac {x}{y}$

$\Rightarrow \cfrac {2.8}{0.8}=\cfrac {3.5}{1}$

$\Rightarrow 3.5=3.5$

Complete the following table of radicals.
Name of Radical Molecular representation
Sulphate
$N^{3-}$
Nitrate
Bisulphite
$CO_{3}^{2-}$
$OH^{-}$
Phosphate

(1) Sulphate radical is donated by

$SO_4^{2-}$ .

(2) The radical donated by

$N^{3-}$ is known as azide.

(3) Nitrate radical is donated by

${NO_3}^-$ .

(4) Bisulphite radical is donated by

${HSO_3}^-$ .

(5) Radical donated by

${CO_3}^{2-}$ is carbonate .

(6)

$OH^-$ radical is known as hydroxide.

(7) Phosphate radical is donated by

${PO_4}^{3-}$ .

State the difference between cations and anions.

Cations are ions with a net positive charge, meaning they have more protons than electrons. Formed by loss of electrons. Example:

$Ag^+,Na^+$ .

Anions are ions with a net negative charge, which means they have more electrons than protons. Formed by gain of electrons. Example:

$Cl^-, O^{2-}$ .

Explain Dalton's atomic theory.

Dalton’s Atomic Theory:

Matter is made up of indivisible particles known as atoms.
The properties of all the atoms of a given element are the same including mass. This can also be stated as all the atoms of an element have identical mass while the atoms of different elements have different masses.
Atoms of different elements combine in fixed ratios to form compounds.
Atoms are neither created nor destroyed. This implies that during chemical reactions, no atoms are created nor destroyed.
The formation of new products (compounds) results from the rearrangement of existing atoms (reactants).
Atoms of an element are identical in mass, size and many other chemical or physical properties, but atoms of two-different elements differ in mass, size, and many other chemical or physical properties.

Name the Indian sage who first propounded the idea of atom (amu).

The concept of the atom was actually formulated by an Indian sage and philosopher, Acharya Kanad, nearly 600 years ago. He called the particle, parmanu which means atom. He thought the atom was a minute object and was invisible, indestructible, and hence, eternal.

State law of constant properties. Give one example to illustrate this law.

The low of constant properties states that a pure chemical compound always contains same elements combined together in the same definite proportion by weight.

Example :- Proust worked with two samples of cupric carbonate. One is naturally occurring and other is prepared in lab.

He found that composition of elements present in two sample are same.

Cupric carbonate

$Cu$

$C$

$O$

(in lab)

$51.35$

$9.7$

$38.9$

(in nature)

$51.35$

$9.7$

$38.9$

Calculate the molar mass of the following substances
(a) Ethyne $C_2H_2$
(b) Sulphur molecule $S_8$
(c) Phophorous molecule $P_4$
(d) Nitric acid $HNO_3$
(e) Hydrochloride acid $HCl$ .

(a) Molar mass of

$C_2H_2=(2\times\text{atomic mass of C}) + (2\times\text{atomic mass of H}$ )

$=2\times 12+ 2\times 1=26\ u$

(b) Molar mass of

$S_8 = (8\times\text{atomic mass of S}$ )

$=8\times 32=256\ u$

$P_4 = (4\times\text{atomic mass of P}$ )

$=4\times 31=124\ u$

(d) Molar mass of

$HCl = (\text{atomic mass of H}) + (\text{atomic mass of Cl}$ )

$=1 + 35.5 = 36.5\ u$

(e) Molar mass of

$HNO_3 = (\text{atomic mass of H}) + (\text{atomic mass of N}) + (3\times\text{atomic mass of O}$ )

$=1+14+ 3\times 16=63\ u$

Find the molecular mass of:
(a) $H_2S$ (b) $HCl$ (c) $NH_3$ (d) $Cl_2$ (e) $CH_3COOH$ (f) $CH_3CHO$

The molecular mass $H$ is 1 g.

The molecular mass of $S$ is 32 g.

The molecular mass of $Cl$ is 35.5 g.

The molecular mass of $N$ is 14 g.

The molecular mass of $C$ is 12 g.

$H_2S$ is 2 × 1 + 32 = 34 g.

$HCl$ is 1 + 35.5 = 36.5 g.

$NH_3$ is 14 + 3 × 1 = 17 g.

$Cl_2$ is 2 × 35.5 = 71 g.

$CH_3COOH$ is 2 × 12 + 4 × 1 + 2 × 16 = 60 g.

$CH_3CHO$ is 2 × 12 + 16 + 4 × 1 = 34 g.

The molecular weight of hydrogen peroxide is $34$ . What is the unit of molecular weight?

A chemical compound is created when two or more elements bonded together. Hydrogen peroxide is created from hydrogen and oxygen. The chemical formula for hydrogen peroxide is

$H_2O_2$ .

Answer and Explanation:

The molar mass of hydrogen peroxide is

$34.01468 g/mol$ .

To calculate molar mass, you need to know the atomic mass of each of the chemical elements that make up the compound. You also need to know the formula for the compound, so you can identify how many atoms of each element are combined.

Hydrogen has an atomic mass of

$1.00794 g/mol$ and oxygen's atomic mass is

$15.9994 g/mol$ .

There are two atoms of each element in hydrogen peroxide, so multiply these atomic masses by 2, and then add the products together.

$1.00794 g/mol x 2 = 2.01588 g/mol$

$15.9994 g/mol x 2 = 31.9988 g/mol$

When you add these products:

$2.01588 g/mol + 31.9988 g/mol = 34.01468 g/mol$

Why is it not possible to see an atom with naked eyes?

State the various postulates of Dalton's atomic theory of matter.

John Dalton, a British school teacher, published his theory about atoms in 1808. His findings were based on experiments and the laws of chemical combination.

Postulates

1. All matter consists of indivisible particles called atoms.

2. Atoms of the same element are similar in shape and mass but differ from the atoms of other elements.

3. Atoms cannot be created or destroyed.

4. Atoms of different elements may combine with each other in fixed, simple, whole-number ratios to form compound atoms.

5. Atoms of the same element can combine in more than one ratio to form two or more compounds.

6. The atom is the smallest unit of matter that can take part in a chemical reaction.

Write symbol of silver.

Silver is a chemical element with the symbol

$Ag$ (from the Latin Argentum, derived from the Proto-Indo-European: "shiny" or "white") and atomic number 47. A soft, white, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal.

Write the symbol of gold.

The chemical symbol of gold is Au, taken from the Latin word for gold - Aurum.

Calculate the formula unit mass of $CaSO_{4.}2H_{2}O$ .

Formula unit mass of

$CaSO_4.2H_2O$

$=$ (Number Calcium atoms x atomic mass of Calcium) + (Number Sulphur atoms x atomic mass of sulphur) + (Number Oxygen atoms x atomic mass of Oxygen) + (Number Water molecules x Molecular mass of water molecule)

$= (1\times 40) + (1\times 32)+(4\times 16) + (2\times 18)$

$= 40 + 32 + 64 + 36$

$=172$

Formula unit mass of gypsum = 172

Write symbol of copper.

The chemical symbol of copper is

$Cu$ ( Pronounced Cee-Yoo not KU)

It has an atomic number of 29 in the periodic table which gives a distinct identity to the chemical element.

Copper is a widely used mineral resource in diverse countries of the world e.g The statue of liberty was made from 80 tons of hammered copper sheet to produce the edifice.

0.607 g of the silver salt of a tribasic acid on combustion deposited 0.37 g of pure silver Calculate the molecular weight of the acid.

$Ag_3A \longrightarrow 3Ag + A$

$3$ moles of $Ag$ combine with $1$ mole of $A$

Mass of $Ag = 0.37gm$

Mass of $A = 0.607 - 0.37 = 0.237gm$

Moles of $Ag = \frac{0.37}{108} = 0.003426$

Moles of $A = \frac{0.003426}{3} = 0.001142$

Molar mass of $A = \frac{0.237}{0.001142} = 207.53gm$

A tribasic acid has $3$ atoms of $H$

Molar mass of the acid $= 3 + 207.53 = 210.53 g/mol$

What is the symbol of the species with number of electrons equal to $36$ protons equal to $35$ and neutrons equal to $45$ ?

Atomic number

$=$ Number of protons

$=35$

Atomic mass

$=35+45=80$

Thus the species is

${_{35}{Br}^{80}}$ .

since the no. of electrons is greater than no. of protons by

$1$ , i.e., the given species has a negative charge.

$\therefore$ Symbol is

${Br}^{-}$

Which postulate of Dalton's atomic theory can explain law of definite proportions?

Law of definite proportion states that "the elements are always present in definite proportions by mass."

And we know that the last postulate of Dalton's atomic theory which states that, "In a given compound, the relative number and types of atoms are constant."

This postulate can explain the law of definite proportions.

Complete the following table.
Atomic
Number Mass
Number
Number
of
Nutrons
Number
of
Protons Number
of
Electrons
Name of the
Atomic Species
9 - 10 - - -
16 32 - - - Sulphur
- 24 - 12 - -
- 2 - 1 - -
- 1 0 1 0 -

Atomic Number	Mass Number	Number of Neutrons	Number of Protons	Number of Electrons	Name of Atomic species
9	19	10	9	9	Fluorine
16	32	16	16	16	Sulphur
12	24	12	12	12	Magnessium
1	2	1	1	1	Deuterium
1	1	0	1	1	Hydrogen

Atomic no

$.=$ No

$.$ of electron

$=$ Number of proton

$.$

$.$ of neutron

$=$ Mass No

$.-$ Atomic no

$.$

Give postulates of Dalton's atomic theory.

Postulates of Dalton's Atomic Theory:

All matter consists of indivisible particles called atoms.
Atoms of the same element are similar in shape and mass but differ from the atoms of other elements.
Atoms cannot be created or destroyed.
Atoms of different elements may combine with each other in a fixed, simple, whole number ratios to form compound atoms.
Atoms of the same element can combine in more than one ratio to form two or more compounds.
The atom is the smallest unit of matter that can take part in a chemical reaction.

Write the names of the following compounds and deduce their molar masses.
${ N }a_{ 2 }{ SO }_{ 4 },\quad { K }_{ 2 }{ CO }_{ 3 },\quad { CO }_{ 2 },\quad { MgCl }_{ 2 },\quad NaOH,\quad { AlPO }_{ 4 },\quad { NaHCO }_{ 3 }$

$Na_{2}SO_{4}$ : Sodium sulphate

Molar Mass :

$23(2) + 32 + 16(4)$ =

$46 + 32 +64$ =

$142$ g/mol

$K_{2}CO_{3}$ : Potassium carbonate

Molar Mass :

$39(2) + 12 + 16(3)$ =

$78 + 12 + 48$ =

$138$ g/mol

$CO_{2}$ : Carbon dioxide

Molar Mass :

$12 + 16(2)$ =

$12 + 32$ =

$44$ g/mol

$MgCl_{2}$ : Magnesium chloride

Molar Mass :

$24+ 35.5(2)$ =

$95$ g/mol

$NaOH$ : Sodium hydroxide

Molar Mass :

$23 + 16 + 1$ =

$40$ g/mol

$AlPO_{4}$ : Aluminium phosphate

Molar Mass :

$27 + 31 + 16(4)$ =

$27 + 31 +64$ =

$122$ g/mol

$NaHCO_{3}$ : Sodium bicarbonate

Molar Mass :

$23 + 1 + 12 + 16(3)$ =

$23 + 1 +12 + 48$ =

$84$ g/mol

How does the Modern atomic theory contradict and correlate with Dalton's atomic theory?

The latest research on the atom has proved that most of the postulates of Dalton's atomic theory contradict. However, Dalton was the right atom to take part in chemical reactions.

Comparisons of Dalton's atomic theory with the modern atomic theory.

$\text{Dalton's atomic theory:}$

∙ Atoms are invisible.
∙ Atomic of the same element is similar in every respect.
∙ Atoms combine in a simple whole-number ratio to form molecules.
∙ Atoms of different elements are different.
∙ Atomic can neither be created nor be destroyed.

$\text{Modern atomic theory:}$

∙ Atoms are divisible and consist of electrons. protons neutrons and even more sub-particle.

∙ Atoms of the same element may differ from one another.

∙ Atoms of different elements may be similar called isobars.

∙ Atoms combine in a ratio which is not a simple whole-number ratio; e.g. in sugar, the

$C_{12}H_{22}O_{11}$ ratio is not a whole number ratio.

Name the reaction in which one of the reacting species of loses electrons, while the other gains electrons.

A reaction in which one of the reacting species loses electrons while the other gains electrons are known as a reduction-oxidation reaction.

For example, $H_{2} + F_{2} \rightarrow 2HF$

Oxidation: $H_{2} \rightarrow 2H^{+} + 2e^{-}$

Reduction : $F_{2} + 2e^{-} \rightarrow 2F^{-}$

Net reaction : $H_{2} + F_{2} \rightarrow 2H^{+} + 2F^{-} \rightarrow 2HF$

An atom of an element is denoted by a "symbol"- Explain the meaning of the term 'symbol'. State a reason for representing the following elements by their symbols.
(a) Hydrogen by 'H'
(b) Helium by 'He'
(c) Copper by 'Cu'

$\text{Symbol:}$ The short form or abbreviated name of the element (or radicals) The reason for representing the following elements by their symbols are as follow :

(a) Hydrogen by 'H' - First letter of the name of the element
(b) Helium by 'He' - First two letters of the name of Noble gases
(c) Copper by 'Cu' - Deriving symbols from their Latin names

Calculate the molecular masses of $H_{2},O_{2},Cl_{2},CO_{2},CH_{4}, C_{2}H_{6},C_{2}H_{4},NH_{3},CH_{3}OH$ .

Molecular mass of the given molecules are calculated below:-

$H_2$ =

$2\times 1 = 2\ g/mol$

$O_2$ =

$2\times 16 = 32\ g/mol$

$Cl_2$ =

$2\times 35.5 = 71\ g/mol$

$CO_2$ =

$1 \times C + 2 \times 16 = 44\ g/mol$

$CH_4$ =

$1 \times 12 + 4\times 1 = 16\ g/mol$

$C_2H_6$ =

$2\times 12 + 6\times 1 = 30\ g/mol$

$C_2H_4$ =

$2\times 12 + 4\times 1 = 28\ g/mol$

$NH_3$ =

$1\times 14 + 3\times 1 = 17\ g/mol$

$CH_3OH$ =

$1\times 12+ 4\times 1 + 1 \times 16 = 32\ g/mol$

What are the ions present in this compound?
$Al_2(SO_4)_3$

The given compound is an aluminium sulphate.

It contain two ions which are

$Al^{3+}$ and

${SO_4}^{2-}$ .

Apply the law of constant proportions to calculate the mass of oxygen that will be used up for combustion of 5 g of ${H_2}$ gas.

The law of constant proportions states that in a chemical substance, the elements are always present in definite proportions by mass.

Combustion of

$H_2$ gas produces steam.

$H_2 + \frac {1}{2} O_2 \rightarrow H_2O$

We can write the following masses of

$H_2$ ,

$O_2$ and

$H_2O$ for the reaction:

$H_2 + \frac {1}{2} O_2 \rightarrow H_2O$

2 g 16 g 18g

According to the reaction, 2 g of

$H_2$ for combustion requires 16 g of

$O_2$

1 g of

$H_2$ will require

$\dfrac {16}{2}$ g of

$O_2$

$\therefore$ 5 g of

$H_2$ requires

$\dfrac {16}{2} \times$ 5 g of

$O_2$ = 40 g

How many atoms of element 35 can combine with ab atom of element 20?

$35 \to Br \to B{r^ - }$

$20 \to Ca \to C{a^{2 + }}$

They form

$\to CaB{r_2}$

State the law of definite proportions. Explain it with the help of an example

1351783_1266846_ans_8814919cdc034aaf95a11b8652f3aaff.jpg

Calculate the number of moles of the following:
(I) $52g$ of $He$

Given that;

Mass of

$He = 52g$

$\therefore$ Number of moles of

$He = \dfrac {Mass \ of \ He}{Molar \ mass \ of \ He} = \dfrac{52}4 \ moles = 13 \ moles$

What are ions? Give some examples.

An ion is an atom or group of atoms where the number of electrons is not equal to the number of protons. That means when a stable atom gains or loses an electron, it becomes an ion.

Examples of ions are as follows:

$H^+,Na^+,Ca^{2+},F^-,O^{2-}$

Is it possible to see atoms these days? Explain your answer.

Yes, it is possible to see atoms these days, with the help of advanced technology. Electronic microscopes are extremely powerful and can magnify even the smallest particle.

What are the main postulates of Dalton's atomic theory? What were it's limitations.

1326108_1316885_ans_65de25df2140451c9e1f2a593c5b69b4.jpg

Water $(H_2O$ ) obtain from any source contains hydrogen and oxygen in the ratio 1:8 by mass. Which Law follows it? Define the Law?

1296305_1353330_ans_db7308add39f4dbea64373b7c44fc546.jpg

(a) State six postulates of Daltons atomic theory.
(b) A 0.24g Sample of compound of boron and oxygen on analysis was found to contain 0.096g of boron and 0.144g of oxygen. Find the percentage composition of the compound by wight.

$\textbf{Part a:}$

Six Postulates of Dalton's atomic theory:

I. All matter is made of very tiny particles called atoms, which participate in chemical reaction.

II. Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.

III. Atoms of a given element are identical in mass and chemical properties.

IV. Atoms of different elements have different masses and chemical properties.

V. Atoms combine in the ratio of small whole numbers to form compounds.

VI. The relative number and kinds of atoms are constant in a given compound.

$\textbf{Part b:}$

Mass of compound = 0.24 g

Mass of boron in the compound = 0.096 g

Mass of oxygen in the compound = 0.144 g

Percentage composition: =

$\frac{Mass\ of\ the\ element}{Mass\ of\ the\ compound}\times100$

Percentage composition of Boron:

Percentage composition of Boron =

$\frac{0.096}{0.24}$ X 100 = 40%

Percentage composition of Oxygen:

Percentage composition of Oxygen =

$\frac{0.144}{0.24}$ X 100 = 60%

Hence, Percentage composition of the sample is:

Boron = 40%

Oxygen = 60%

Verify by calculation that 5 moles of $CO_2$ and 5 moles of $H_2O$ not have same mass.

1286310_1345888_ans_81402d90821e4d26b6b1b46809ea3182.jpg

Explain the mole concept. What is the importance of mole concept.

MOLE CONCEPT

$\bullet$ The mole is an unit of amount.

$\bullet$ It is defined as the mass of substance containing the same number of fundamental units.

$\bullet\ 1$ mole

$= 6.022 \times 10^{23} = N_{A}$ (particles)

= Avogadro Number

IMPORTANCE:-

$\bullet$ It is important because it allows chemist to work with micro units and amounts.

$\bullet$ You can easily change mole to gram or gram to mole.

State law of definite proportions.

The Law of definite proportion states, that in a given compound, elements are combined in a fixed ratio by mass.

Differentiate between ${\text{2H}}$ and ${{\text{H}}_{\text{2}}}$ .

2041248_1360539_ans_f908f0c273194e56a89b150bf89c601e.jpg

Mass of 1.5 mole of ${ CO }_{ 2\ }$ molecule

$\textbf{Given that:}$

The number of moles of

$CO_{2} = 1.5\ mole$

$\textbf{Step 1:}$ Determine the molecular mass of

$CO_{2}$

The molecular mass of

$CO_{2}$ can be calculated as,

$CO_{2}=(1\times C)+(2\times O)$

$=12+2\times16=44$

Thus, the molecular mass of

$CO_{2}$ is 44.

$\textbf{Step 2:}$ Determine the mass of 1.5 mole of

$CO_{2}$

The mass of

$CO_{2}$ can be calculated as shown below.

$Number \ of \ moles=\dfrac{Given \ mass}{Molecular \ mass}$

$1.5=\dfrac{Given \ mass}{44}$

$Given \ mass=1.5\times 44=66 \ g$

Therefore, the mass of

$CO_{2}$ is

$66 \ g.$

Calculate the mass of the following
(i) $0.5$ mole of $N_{2}$ gas (mass from mole of molecule)
(ii) mole of $N$ atoms (mass from mole of atom)
(iii) $3.011\times 10^{23}$ number of N atoms (mass from number)
(iv) $6.022\times 10^{23}$ number of $N_2$ atoms (mass from number)

(i)

Number of moles

$=0.5$

Molar mass of

$N_2=14\times 2$

$=28$

Since,

$Mass=Molar\ mass\times number\ of\ moles$

Therefore,

$Mass=28\times 0.5$

$=14$

Hence, the mass of

$0.5$ mole of

$N_2$ is

$14g$

(ii)

Number of moles

$=0.5$

Molar mass of

$N=14$

Since,

$Mass=Molar\ mass\times number\ of\ moles$

Therefore,

$Mass=14\times 0.5$

$=7$

Hence, the mass of

$0.5$ mole of

$N$ is

$7g$

(iii)

Number of N atoms

$=3.011\times 10^{23}$

Since,

$1\ mole=6.022\times 10^{23}\ atoms$

Therefore,

$3.011\times 10^{23} atoms=\dfrac{3.011\times 10^{23}}{6.022\times 10^{23}} number\ of\ moles$

So,

$3.011\times 10^{23}$ number of

$N$ atoms has

$0.5$ mole of

$N$ atoms.

Since,

$Mass=Molar\ mass\times number\ of\ moles$

Molar mass of

$N=14$

Therefore,

$Mass=14\times 0.5$

$Mass=7g$

Hence, the mass of

$3.011\times 10^{23}$ number of

$N$ atoms is

$7g$

(iv)

Number of

$N_2$ atoms

$=6.022\times 10^{23}$

Since,

$1\ mole=6.022\times 10^{23}\ atoms$

Therefore,

$6.022\times 10^{23} atoms=\dfrac{6.022\times 10^{23}}{6.022\times 10^{23}} number\ of\ moles$

So,

$6.022\times 10^{23}$ number of

$N_2$ atoms has

$1$ mole of

$N_2$ atoms.

Since,

$Mass=Molar\ mass\times number\ of\ moles$

Molar mass of

$N_2=28$

Therefore,

$Mass=28\times 0.5$

$Mass=14g$

Hence, the mass of

$6.022\times 10^{23}$ number of

$N_2$ atoms is

$14g$

Which postulate of Dalton's atomic theory is the result of the law of conservation of mass?

The postulate of Dalton’s atomic theory resulting from the law of conservation of mass is ‘The relative number and kinds of atoms are constant in a given compound. Atoms cannot be created nor be destroyed in a chemical reaction’.

How to calculate the molar mass?

Molar mass is the mass (in grams) of one mole of a substance.

Molar mass of a compound can be calculated by adding the standard atomic masses (in g/mol) of the constituent atoms.

For example: Molar mass of

$H_2O$

= 2

$\times$ atomic mass of hydrogen + 1

$\times$ atomic mass of oxygen= 2

$\times$ 1 +1

$\times$ 16= 18

$g/mol$

What is the weight of $3.01 \times 10^{23}$ molecules of ammonia?

$\because$

$6.023 \times 10^{23}$ molecules of

$NH_3$ has weight

$= 17\,g$

$\therefore$

$3.01 \times 10^{23}$ molecules of

$NH_3$ has weight

$= \dfrac {17 \times 3.01 \times 10^{23}}{6.023 \times 10^{23}} = 8.50\,g$

Which postulate of Dalton's atomic theory is the result of law of conservation of mass given by Lavoisier?

The postulate of Dalton's atomic theory "Atoms can neither be created nor destroyed" is the result of the law of conservation of mass given by Lavoisier.

Which part of Dalton's atomic theory came from the law of constant proportions given by Proust?

The elements consist of atoms having fixed mass , and that the number and kind of atoms of each element in a given compound is fixed.

What is the scientific name of particles which make up matter?

Atoms or molecules make up matter or matter is composed of atoms/molecules.

Calculate the molecular masses of the following compounds:
(a) Methanol $CH_3OH$
(b) Ethanol, $C_2H_5OH$

(a) Molecular mas of methanol

$(CH_3OH) = (1 \times C) + (3 \times H) + (1 \times O) + (1 \times H) = 12 + (3 \times 1) + 16 + 1 = 32\ u$

(b) Molecular mass of ethanol

$(C_2H_5OH) = (2 \times C) + (5 \times H) + (1 \times O) + (1 \times H) = (2 \times 12) + (5 \times 1) + 16 + 1 = 46\ u$

Define 'molecular mass' of a substance.

The molecular mass of a substance is the relative mass of its molecule as compared with the mass of a Carbon-

$12$ atom taken as

$12$ units.

What is meant by saying that ' the molecular mass of oxygen is $32$ '?

This means that a molecule of oxygen is

$32$ times heavier than

$1/12$ of a Carbon-

$12$ atom.

Calculate the molecular masses of the following compounds :
(a) Methane , $CH_4$
(b) Ethane , $C_2H_6$
(c) Ethene , $C_2H_4$
(d) Ethyne , $C_2H_2$
(Atomic masses : $C =12u , H = 1u$ )

(a) Molecular mass of methane ,

$(CH_4) = 12 + 4 = 16u$

(b) Molecular mass of ethane ,

$(C_2H_6) = 2 \times 12 + 6 \times 1 = 30u$

$(C_2H_4) = 2 \times 12 + 4 \times 1 = 28u$

(d) Molecular mass of ethyne

$( C_2H_2) = 2 \times 12 + 2 \times 1 = 26u$

The molecular formula of glucose is $C_6H_{12} O_6 .$ Calculate its molecular mass ( Atomic mass : $C = 12 u : H = 1u ; O= 16u$ )

Molecular formula of glucose is

$C_6H_{12}O_6$ .

Atomic mass of

$C = 12u$

Atomic mass of

$H = 1u$

Atomic mass of

$O = 16 u$

Molecular mass =

$(6 \times12) + (12 \times 1) + (6 \times 16) = 180u$

Calculate the molecular mass of chloroform $(CHCl_3)$ .
(Atomic masses: $C = 12u ; H = 1u ; Cl = 35.5u$ )

Molecular mass of chloroform

$(CHCl_3) = 1 \times C + 1 \times H + 3 \times Cl = (12 + 1 + 106.5)u = 119.5 \,u$

(a) Name the element used as a standard for atomic mass scale.
(b) Which particular atom of the above element is used for this purpose?
(c) What value has been given to the mass of this reference atom?

(a) Carbon is used as a standard for atomic mass scale.

(b) Atom with

$6$ neutrons and

$6$ protons in its nucleus so that its mass number is

$12$ , i.e. a carbon-12 atom is used for this purpose.

$12\ u$ .

Calculate the molecular masses of the following :
(a) Hydrogen , $H_2$
(b) Oxygen , $O_2$
(c) Chlorine , $Cl_2$
(d) Ammonia , $NH_3$
(e) Carbon dioxide , $CO_2$
(Atomic masses : $C = 12 u ; H = 1u ; O = 16 u ; Cl = 35.5 u ; N = 14u$ )

(a) Molecular mass of Hydrogen

$(H_2) = 2 \times H = 2 \times 1 \, u = 2 \,u$

(b) Molecular mass of Oxygen

$(O_2) = 2 \times O = 2 \times 16\,u = 32\,u$

$(Cl_2) = 2 \times Cl = 2 \times 35.5 = 71 \,u$

(d) Molecular mass of Ammonia

$(NH_3) = 1 \times N + 3 \times H = 14 + 3 = 17\,u$

(e) Molecular mass of Carbon dioxide

$(CO_2) = 1 \times C + 2 \times O = 12 + 32 = 44 \,u$

(a) What is an atom? How do atoms usually exist?
(b) What is a molecule? Explain with an example.
(c) What is the difference between the molecule of an element and the molecule of a compound? Give one example of each.

(a) Atoms are building blocks of all matter. They are the smallest particles of elements that can take part in a chemical reaction. Atoms generally exist as molecules of elements and molecules of compounds.

(b) A molecule is made up of atoms and it is the smallest particle of a substance which has the properties of that substance.

Example: Oxygen atoms combine to form

$O_3$ molecules

(c) Molecule of an element consists of two or more atoms of the same element chemically bonded together. On the other hand, a molecule of a compound consists of two or more atoms of different elements chemically bonded together.

Hydrogen molecule is an example of molecule of an element. In it, two

$H$ atoms are bonded to give a molecule of hydrogen

$H_2$ .

Example for molecule of a compound is nitric acid, in which one atom of hydrogen, one atom of nitrogen and three atoms of oxygen are chemically bonded together to form

$HNO_3$ .

Give reasons for the following:
Atoms of most elements cannot exist independently, then how do these atoms form the matter that we can feel, see, or touch?

Atoms of most elements are not able to exist independently. Atoms form molecules and ions. These molecules or ions aggregate in large numbers to form the matter that we can see, feel or touch.

What is meant by molecular mass?

The molecular mass of a substance is the sum of atomic masses of all the atoms in a molecule of the substance. It is used for those substances whose constituent particles are molecules.

For example:

The molecular mass of

$CO_2 = (1 \times atomic\ mass\ of\ carbon) + ( 2 \times atomic\ mass\ of\ oxygen)$

$\Rightarrow 1\times12+2 \times 16$

$\Rightarrow 12+ 32 = 44\ u$

Give reasons for the following:

Give the difference between an atom and an ion.

Atoms are neutral since they have an equal number of protons and electrons. While ions are charged species produced by either losing or gaining electrons.

Calculate the molecular masses of the following compounds:

(a) Hydrogen sulphide, $H_2S$
(b) Carbon disulphide, $CS_2$

(Atomic masses: H = 1u , S = 32u, C = 12u)

(a) Molecular mass of hydrogen sulphide,

$(H_2S) = (2 \times 1) + (1 \times 32) = ( 2 + 32)u = 34 u$

(b) Molecular mass of carbon disulphide,

$(CS_2) = (1\times 12) + (2 \times 32) = ( 12 + 64)u = 76u$

Calculate the molecular mass of hydrogen bromide $(HBr)$ .
(Atomic masses: $H = 1u ; Br = 80u$ )

Molecular mass of hydrogen bromide

$(HBr) = 1\times H + 1 \times Br = ( 1+ 80)u = 81\,u$

State the law of constant proportions . Give one example to illustrate this law.

Joseph Proust put forward law of proportion in

$1779.$ This law was stated by Proust as “In a chemical substance the elements are always present in definite proportions by mass”.

For example- Carbon dioxide exists in nature in different forms and can be chemically produced in different ways. Yet, all the different samples of carbon dioxide will always contain the same elements. i.e. carbon and oxygen is in the same proportion by mass

$3:8.$

Why is atom considered as a neutral particle?

There are three subatomic particles in an atom which are the electrons, protons, and neutrons. Neutron is electrically neutral and proton is positively charged and an electron is negatively charged. In an atom, the number of protons is equal to the number of electrons. Hence, the overall charge in an atom is zero and it is a neutral particle.

Explain giving a suitable example: Law of constant proportion.

The law of constant proportion states that in a chemical substance, the elements are always present in definite proportions by mass.

For example:

In water,

$H_2O$ , hydrogen, and oxygen are always present in the ratio of 1:8 by mass, whatever the method or source from which water is obtained.

Mass of 2 hydrogen atom=2

Mass of one oxygen atom=16

Hence, the ratio of hydrogen and oxygen by mass=

$2:16$ =

$1:8$

Give the main postulates of Dalton's atomic theory.

Postulates of Dalton's atomic theory are:

i. Elements are made up of very small indivisible particles called atoms.

ii. All atoms of a particular element are identical in shape.

iii. The properties of atoms of a particular element are different from the atoms of other elements.

iv. An atom neither be created nor destroyed.

v. Atoms of one element cannot be converted into those of another element.

Define the following term:
Avogadro's number

Avogadro's number is the number of particles (atoms, ions or ions) present in one mole of any substance is fixed with a value of

$6.022 \times 10^{23}$ . This is an experimentally obtained value.

The chlorine atom is electrically neutral but chloride ion is charged . Explain .

Chlorine atomic number is

$17$ so its number of protons = number of electrons =

$17$ so chlorine atom is electrically neutral. But when chlorine atom is changed to chloride ion it has gained one electron so the number of electrons is more than a number of protons hence it is negatively charged.

Give the symbols for:
Beryllium, Barium, Radium, Sodium

The symbols for these elements are:

Beryllium - Be
Barium - Ba
Radium - Ra
Sodium - Na

Give the symbols for:
Two non-metals of group $16$

Group

$16$ family is also known as oxygen family. Name and symbols for two non metal from this family is mentioned below.

Element (Non-metal) Symbol

1) Oxygen O

2) Sulphur S

Name the smallest particle of an element that can retain all the chemical properties.

An atom is the smallest particle of an element that can retain all the chemical properties. It is the indivisible unit of matter that can exist independently. It is a building block of matter.

Size of the nucleus of an atom is _______ as compared to the size of the atom .

Size of the nucleus of an atom is very small as compared to the size of the atom.

According to Rutherford gold foil experiment, nucleus is very small in size as compared to the size of the atom as a whole. Nucleus is very hard, dense and positively charged which consists of protons and neutrons.

Find the ratio by mass of the combining elements in the following compound:
$H_2SO_4$

Molar mass of hydrogen

$= 1 g$

Molar mass of sulphur

$= 32 g$

Molar mass of oxygen

$=16 g$

Hence, the ratio by mass

$= (2\times H):S: (4\times O)$

$= (2\times 1) : 32 : ( 4 \times 16)$

$= 2:32:64$

On simplifying, we get

$1:16:32$

Find the ratio by mass of the combining elements in the following compound:
(I) $Ca(OH)_2$

Mass of Ca : Mass of

$O_2$ : Mass of

$H_2 = 40 : (16 \times 2) : 1 \times 2 = 40 : 32 : 2 = 20 : 16 : 1$

Find the ratio by mass of the combining elements in the following compound:
(I) $NH_3$

Mass of

$N$ : Mass of hydrogen

$\times$ number of atoms of hydrogen

$= 14 : (1 \times 3) = 14 : 3$

Find the ratio by mass of the combining elements in the following compound:
$C_2H_5OH$

Atomic masses of

$C, H \ and \ O$ are 12, 1 and 16 a.m.u respectively

Mass of

$C_2$ : Mass of

$H_6$ : Mass of

$O$

$= (12 \times 2) : (1 \times 6) : (16 \times 1) = 24 : 6 : 16 = 12 : 3 : 8$

Find the ratio by mass of the combining elements in the following compound:
$MgCl_2$

Atomic mass of

$Mg$ = 24 u
Atomic mass of

$Cl$ = 35.5 u
Ratio of mass of combining elements in

$MgCl_2$ is given by:
Mass of

$Mg$ : Mass of

$Cl_2=24:(35.5\times 2)=24:71$

Write the cations and anions present(if any) in the following compounds:
(a) $CH_3COONa$
(b) $NaCl$
(c) $H_2$
(d) $NH_4NO_3$

1651344_1791530_ans_8cd6800b086c4c0eaee93012e558c452.jpg

Verify by calculating that $240$ g of calcium and $240$ g of magnesium have a mole ratio of $3:5$ .

Number of moles in

$240g$ Ca metal

$=\dfrac{240}{40}=6$
Number of moles in

$240g$ of Mg metal

$=\dfrac{240}{24}=10$
Ratio$$=6:10$$
Ratio$$=3:5$$

Find the ratio by mass of the combining elements in the following compound:
$CaCO_3$

Atomic mass of

$Ca$ = 40 u

Atomic mass of

$C$ = 12 u

Atomic mass of

$O$ = 16 u
Rati of of mass of combining elements in

$CaCO_3$ is given by:
Mass of

$Ca$ : mass of

$C$ : mass of

$O_3 =40:12:16\times3=40:12:48=10:3:12$

Write any two observations which support the fact that atoms are divisible.

Discovery of electrons and the discovery of protons show that atoms are made up of even smaller particles. During a chemical reaction, there is either transfer of electrons or sharing of electrons between different atoms which leads to the rearrangement of atoms. These two observations support the fact that atoms are divisible.

Fill in the missing data in Table.

The missing data in the table is as follows:-
1652798_1793081_ans_4e44a9d6dfcf46eb81cc1e8d47407fb7.jpg

What is the symbol for SI unit of mole? How is the mole defined?

Symbol for SI Unit of mole is mol.
One mole is defined as the amount of a substance that contains as many particles or entities as there are atoms in exactly

$12\ g(0.012\ kg)$ of the

$C-12$ isotope.

Name the scientists who discovered the following:
Atoms

Atoms: John Dalton was a British chemist and physicist. He proved that matter consists of small indivisible particles called 'atoms'. For this, he proposed the atomic theory which was later on called 'Dalton's atomic theory.

The visible universe is estimated to contain $10^{22}$ stars. How many moles of stars are present in the visible universe?

$1$ mole of stars equals

$6.023 \times 10^{23}$

$\therefore$ Number of moles of stars

$=\dfrac {10^{22}}{6.023 \times 10^{23}}= 0.0166\,moles$

What will be the mass of one atom of $C-12$ in grams?

Mass of

$6.022\times 10^{23}$ atoms of

$C$ in

$gm=12\ g$
Mass of one atom of

$C=12/6.022\times 10^{23}=1.992648\times 10^{-23}\ g$

State three characteristics of molecules of matter.

Three characteristics of molecules of matter are:
1. They are very small in size.
2. They have space between them.
3. They are in constant random motion.

Write the chemical names of the following and also give their molecular formulae:
(a) Baking soda
(b) Vinegar
(c) Marble
(d) Sand

(a) Sodium bicarbonate (Baking soda)

$- NaHCO_3$

(b) Acetic acid (Vinegar)

$- CH_3COOH$

$- CaCO_3$

(d) Silicon dioxide (Sand)

$- SiO_2$

Atomic theory of matter was given by ..............?

Atomic theory of matter was given by

$\underline{John\ Dalton}$ . He postulated that all matter, whether an element, a compound or a mixture is composed of small particles called atoms.

Mention briefly the salient features of Dalton's atomic theory (five points).

Salient features of Dalton's atomic theory:

1. Matter consists of very small and indivisible particles called atoms, which can neither be created nor can be destroyed.

2. The atoms of an element are alike in all respects i.e. size, mass, density, chemical properties but they differ from the atoms of other elements.

3. Atoms of an element combine in small numbers to form molecules of the element.

4. Atoms of one element combine with atoms of another element in a simple whole-number ratio to form molecules of compounds.

5. Atoms are the smallest units of matter that can take part in a chemical reaction during which only the rearrangement of atoms takes place.

Fill in the blanks.
An ion which has a positive charge is called a ................

An ion which has a positive charge is called a

$\textbf{cation.}$

Fill in the blank:
The ......... of an atom is very hard and dense.

The nucleus of an atom is very hard and dense.

What are the two main parts of which an atom is made of?

1. The centrally located nucleus.

2. The outer orbits in which electrons are present.

What do the following symbol stands for:
$2H$

$2H$ stands for

$2$ atoms of Hydrogen

What is the contribution of the following scientist in atomic structure?
(I) Democritus

Democritus's greatest contribution to modern science was arguably the atomic theory he elucidated. According to Democritus' atomic theory, the universe and all matter obey the following principles:

Everything is composed of “atoms”, which are physically, but not geometrically, indivisible.

Define Cation and Anion.

Cation: Atoms that carry positive charges are called cations.
Example;

$Na^+$
Anion: Atoms which carry negative charge are called anions.
Example;

$Cl^-$

What is a symbol? What information does it convey?

Short form of atom of specific element or the abbreviations used to refer names of the element is known as symbol.
1. It represents a specific element.
2. It represents one atom of an element.
3. A symbol represents how many atoms are present in its one gram (gm) atom.
4. It represents the number of times an atom is heavier than one atomic mass unit (amu) taken as a standard.

What is the contribution of the following in Atomic structure.
Maharshi Kanada

Maharshi Kanada has postulated that if we go on the dividing matter, we shall get the smaller and smaller particle of matter. If we continue with dividing matter ultimately a stage will come when we shall come across the smallest particle of matter which can not be divided further. He called these smallest particles as

$\text{parmanu}$ . Today we called these particles atoms.

Fill in the blanks:
Negatively charged ions are called __________ .

A negatively charged ions are called

$anions$ . Some examples of anions are

$Cl^-,\ SO_4^{2-},\ NO_3^-,$ etc.

What does the following symbol stand for?
$2H_2$

The symbol

$2H_2$ stands for

$2$ molecules of Hydrogen.

Explain with examples what is meant by a 'mole' of a substance.

One mole is defined as the amount of substance containing as many elementary entities (atoms, molecules, ions, electrons, radicals, etc.) as there are atoms in

$12$ grams of carbon -

$12 (6.023 \times 10^{23})$ .

The mass of one mole of a substance equals to its relative molecular mass expressed in grams. Mole defines the quantity of a substance. One mole of any substance will always contain

$6.022 \times 10^{23}$ particles, no matter what that substance is.

Therefore, we can say:

$1$ mole of potassium atoms

$(K)$ contains

$6.022 \times 10^{23}$ potassium atoms.

$1$ mole of potassium ions

$(K^+)$ contains

$6.022 \times 10^{23}$ potassium ions.

$1$ mole of hydrogen atoms

$(H)$ contains

$6.022 \times 10^{23}$ hydrogen atoms.

$1$ mole of hydrogen molecules

$(H_2)$ contains

$6.022 \times 10^{23}$ hydroger molecules.

The number of particles that are contained within one mole of a substance is called ________

Avogadro's number which is equal to

$6.023 \times 10^{23}$

Define the terms: cation

Positively charged ions that are formed after losing an electron is called a cation. For example, Sodium loses an electron and forms

$Na^+$ cation.

Define the term anion.

Negatively charged ion formed after gaining an electrons is called anion.

Example:

$Cl + e^- \rightarrow Cl^-$

State postulates of Dalton's atomic theory.

Dalton’s atomic theory states that all matter, whether an element, a compound or a mixture is composed of small particles called atoms. The postulates of this theory are as follows:

All matter is made of very tiny particles called atoms, which participate in chemical reactions.
Atoms are indivisible particles, which cannot be created or destroyed in a chemical reaction.
Atoms of a given element are identical in mass and chemical properties.
Atoms of different elements have different masses and chemical properties.
Atoms combine in the ratio of small whole numbers to form compounds.
The relative number and kinds of atoms are constant in a given compound.

State the difference between cation and anion.

$Cation$	$Anion$
Positively charged ions are called cations. They are formed when atoms lose electrons. For example: In $NaCl$ , $Na^+$ is the cation.	Negatively charged ions are called anions. They are formed when atoms gain electrons. For example: In $NaCl$ , $Cl^-$ is the anion.

Write names of $5$ compounds used in our daily life.

Five compounds used in our daily life are
1. Common salt
2. Water
3. Soap
4. Baking soda
5. Sugar

Utensils of brass and steel, air, glass, $H_2O$ , stone, etc. Are all these made up of one type of substance?

No, all these substances are formed when two or more elements or compounds are mixed in a ratio. For instance, air is a mixture of different elements like oxygen, hydrogen, and compounds like carbon dioxide.

Define element, compound and mixture with examples.

Element: The atoms of an element are similar in their properties. For example iron, copper, aluminum, gold, etc.

Compound: When two or more atoms combine through a chemical reaction in a fixed ratio, a compound is formed. For example, sugar, glass, salt, soap, etc.

Mixture: Mixture is a substance that is formed when two or more compounds or elements are mixed in any ratio. For example- sherbet, air. sand, etc.

Classify elements and mixtures from the following: Copper, brass, hydrogen, sand.

1. Elements - copper, hydrogen (made of only one type of atom)
2. Mixture - brass, sand (mixture of different elements and compounds, both atoms and molecules)

Write the names of five compounds along with their chemical formulae.

Five compounds and their Formula's.

Sr.No	Name of Compound	Formula
1.	Water	$H_2O$
2.	Sodium Chloride	$NaCl$
3.	Carbon dioxide	$CO_2$
4.	Magnesium chloride	$MgCl_2$
5.	Hydrochloric acid	$HCl$

Match the following correctly

a. Iron	3. Element
b. Salt	4. Compound
c. Sherbet	2. Mixture
d. Electron, Proton, Neutron	1. Atom

Explain the atoms present in a water molecule with a diagram.

A molecule of water is formed from two atoms of hydrogen and one atom of oxygen

$(H_2O)$ .
1718514_1819718_ans_cbc94a702c4c48159ca2375e534aafd6.png

Why is Maharshi Kanad famous? How did he get this name?

In the Vedic period, the great philosopher Kanad authored the Veseshik Sutra which is basic nuclear physics for scientists of the West. He propounded the atomic theory of Veseshik philosophy. He gave the concept that atom combines together to make molecules. He ate Tandual particles (Sama rice in Hindi) during his meditation that made his name ‘Kanad’. He was the originator of the particle-molecule theory and hence called Kanad.

Identify elements, compounds, and mixture from the following: oxygen gas, iron, sugar, salt, hydrogen gas, sand, brass, soap, sugar-syrup, sherbet, air.

Element - Oxygen gas. Iron, Hydrogen gas
Compound - Sugar, Salt
Mixture - Sand, Brass, Soap, Sugar- syrup, Sherbet, Air

Give an example of monoatomic molecule.

$Argon$
A monatomic molecule is one in which atoms are not bound to each other.
Noble gases exist as monatomic elements.

Who coined the term 'parmanu' ?

An Indian philosopher Maharishi Kanad, postulated that if we go on dividing matter ('padarth'), we shall get smaller and smaller particles. Ultimately, a stage will come when we shall come across the smallest particles beyond which further division will not be possible. He named these particles 'Parmanu'.

What is an ion?

An atom or group of atoms that carries a positive or negative electric charge as a result of having lost or gained one or more electrons is known as an ion.

What is the approximate size of Parmanu or atom in meters?

The approximate size of Parmanu or atoms is about

$\mathrm10^{-10} m$ .

Define the term atom.

An atom is the smallest particle of an element that can take part in a chemical reaction, it may or may not exist independently.

Why is anion bigger than its corresponding atom?

Anions are negatively charged ions formed when an atom gains electrons. When an atom forms an anion, the number of protons remains the same but the number of electrons becomes more. The repulsive force between the electrons becomes more than the attraction of the positively charged nucleus and as a result, the electron cloud of the anion spreads and becomes bigger in size.

Answer the following questions:
(a) What is meant by mole?
(b) What is the unit of the mole?
(c) What is meant by molar mass?

A mole of a substance is defined as the mass of substance containing the same number of fundamental units as there in

$12$ grams of carbon (i.e.

$6.022\times 10^{23}$ )

Mole is itself the

$SI$ unit of measurement of chemical compounds.

The molar mass of a substance is the total of the atomic masses of the atoms present in that molecule or compound in one mole of molecules or compounds.

Write the name and symbol of some positive and negative ions.

Name of positive ions	Symbols	Name of negative ions	Symbols
Potassium ion	$K^{+}$	Hydroxide ion	$OH^{-}$
Zinc ion	$Zn^{2+}$	Carbonate ion	$CO_{3}^{2-}$
Ferrous ion	$Fe^{2+}$	Bicarbonate ion	$HCO_{3}^{-}$
Ferric ion	$Fe^{3+}$	Nitrate ion	$NO_{3}^{-}$
Cuprous ion	$Cu^{+}$	Sulphate ion	$SO_{4}^{2-}$
Cupric ion	$Cu^{2+}$	Bisulphate ion	$HSO_{4}^{-}$
Ammonium ion	$NH_{4}^{+}$	Phosphate ion	$PO_{4}^{3-}$
Manganese ion	$Mn^{2+}$	Dihydrogen phosphat ion	$H_{2}PO_{4}^{-}$

What do you mean by Avogadro Number ?

The number of molecules present in

$1 \;gm$ molecular weight of any substance.

The number of atoms present in

$1\; gm$ atomic weight of any element. It is represented by the symbol

$N= 6.022 \times 10^{23}$

Compute the mass of one molecule and the molecular mass of $C_6H_6$ (benzene).
(At. mass of $C = 12, H = 1 \,u$ ).

Molecular weight of

$C_6H_6 = 6 \times 12 + 6 \times 1 = 78 \,g,$

Mass of

$1$ molecule

$= \dfrac{78}{6.002 \times 10^{23}} = 12.94 \times 10^{-23} g = 1.294 \times 10^{-22}g$

Which is the positive ion present in sodium hydroxide? Which is the negative ion present in hydrochloric acid?

The chemical formula of sodium hydroxide is

$NaOH$ .

$NaOH \rightarrow Na^+ + OH^-$

Here

$Na^+$ is position ion, which is also called a cation.

The chemical formula of hydrochloric acid is

$HCl$ .

$HCl \rightarrow H^+ +Cl^-$

Here

$Cl^-$ is a negative ion, which is also called an anion.

What is $1$ mole?

$1$ mole is the amount of substance that contains the same number of entities as there are atoms in exactly

$12$ g of the

$^{12}C$ isotope.

$\therefore$ Number of atoms in

$12$ g of

$^{12}C$ - isotope

$=6.023\times 10^{23}$ atoms/mol

The term 'mole' is given by the ''Amedeo Avaogadro''. So it is also known as known as Avogadro's number (

$N_A$ )

$\therefore$

$1 \ mole = N_{A} = 6.023\times 10^{23}$ atoms, molecules, ions etc.

Find out the amount of $CO_{2}$ formed when the burning of one mole of Ethane takes place.

To find out the amount of

$CO_2$ produce we need to write a balanced chemical equation.

The balanced chemical equation for the burning of ethane is,

$2C_2H_6 + 7O_2 \longrightarrow 4CO_2 + 6H_2O$

From the above reaction we know, from

$2$ moles of ethane

$4$ moles of

$CO_2$ are produced so, after burning

$1$ mole of ethane

$2$ moles of

$CO_{2}$ can be produced.

Mass of

$2$ moles of

$CO_{2}=2\ g\times 44\ g/mol=88\ g$

Write assumptions of Dalton's atomic theory.

1. Element is made up of extremely small particles that cannot be seen with naked eyes. These particles are called atoms. Atom is the smallest indivisible particle of an element.

2. All atoms of a given element are the same. Their weights and properties are all alike. The atoms of one element differ from the atoms of other elements and each element has its independent identity.

3. An atom cannot be created or destroyed. Under normal conditions, an atom of one element cannot be changed into an atom of another element.

4. During chemical combinations between two or more elements, atoms combine in a definite proportion. This proportion can be expressed by simple whole numbers. The resulting particles are called molecules and are bigger than combining atoms.

5. Atom is extremely small and indivisible. It maintains its existence during any physical or chemical change. Example - When

$Na$ and

$Cl$ combine,

$NaCl$ will be formed and not

$KCl$ or any molecules with another element.

Define molecules of elements and atomicity.

The molecules of an element are constituted by the same type of atoms. Molecules of many elements, such as argon, helium, etc. are made up of only one atom of that element.

But this is not the case with most of the non-metals. For example, a molecule of oxygen consists of two atoms of oxygen and hence it is known as a diatomic molecule,

$O_2$ . If 3 atoms of oxygen unite into a molecule, instead of the 2, we get ozone,

$O_3$ . The number of atoms constituting a molecule is known as its atomicity

How are molecules and elements formed?

Element is the basic unit of a substance. Two or more than two atoms of the same element or different element combine in a definite proportion to form a molecule.

Example-

1. Two atoms of oxygen combine to form a molecule of oxygen.

2. Two atoms of hydrogen and an atom of oxygen combine to form one molecule of water.

Give two points of the differences:
Atom and Ion

Atom	Ion
An atom is electrically neutral.	An ion has a positive or negative charge.
An atom has the same number of protons and electrons.	An ion has a different number of protons and electrons.

Which theory was proposed by John Dalton?

John Dalton proposed the atomic theory which was based on laws of chemical combination.

Dalton's atomic theory provided an explanation for the law of conservation of mass and the law of definite proportion.

Find molecular masses of $K_2Cr_2O_7$ and $Cr_2(SO_4)_3$ .

Molecular mass of

$K_2Cr_2O_7 =2\times$ (Atomic mass of

$K$ )

$+2\times$ (Atomic mass of

$Cr$ )

$+7\times$ (Atomic mass of

$O$ )

$=2\times(39)+2\times(52)+7\times(16)$

$=78+104+112$

$=294\ amu$

Molecular mass of

$Cr_2 (SO_4)_3=2\times$ (Atomic mass of

$Cr$ )

$+3\times$ (Atomic mass of

$S$ )

$+4\times3\times$ (Atomic mass of

$O$ )

$=2\times(52)+3\times[(32)+4\times(16)]$

$=2(52)+3\times[32+64]$

$=104+3\times[96]$

$=104+288$

$=392\ amu$

Explain:
The basic unit of an element is atom, whereas the basic unit of a compound is molecule.

Elements consist of individual atoms. Each element always consists of the same type of atom which imparts its own unique properties to that element. So, the basic unit of an element is the atom. For example, the element hydrogen consists of hydrogen atoms only.

Compounds consist of the smallest unit which contains at least two different types of atoms join together by chemical bonds; so, the basic unit for the compound is the molecule. For example, water is a compound and is made of molecules (

$H_2O$ ).

Which isotope of carbon is used for getting relative atomic masses?

C-12 is used for getting relative atomic masses.

Tabulate the number of atoms and molecules in following:
$5H_2,2H_2O,3ZnCl_2,CO_2$

1800034_1850180_ans_984b58de9b294e959c94ef6a40e3ecf0.png

Examine the following molecules and write the atoms contained in them.
Carbon dioxide
Hydrogen chloride
Water
Sugar
Mercuric oxide

1. Carbon dioxide chemical formula is

$CO_2$ so it contains

$C$ and

$O$ atom.
2. Hydrochloric acid chemical formula is

$HCl$ so it contains

$H$ and

$Cl$ atom.
3- Water has a chemical formula

$H_2O$ so it contains

$H$ and

$O$ atom.
4. Sugar has a chemical formula

$C_{12}H_{22}O_{11}$ so it contains

$H$ ,

$C$ , and

$O$ atom.
5- Mercuric oxide has a chemical formula

$HgO$ so it contains

$Hg$ and

$O$ atom.

How many hydrogen and sulphur atoms are there in $H_2SO_4$ molecule?

Sulfuric acid contains 2 hydrogen atoms, 1 sulfur atom, and 4 oxygen atoms
hence answer is Hydrogen-2 Sulphar-1

Match the mass of elements given in column $I$ with the no. of moles given in column $II$ and mark the appropriate choice.

$(A): 28\ g$ of

$He=\dfrac {28}{4}=7$ mol

$(B): 46\ g$ of

$Na=\dfrac {46}{23}=2$ mol

$(C): 60\ g$ of

$Ca=\dfrac {60}{40}=1.5$ mol

$(D): 27\ g$ of

$Al=\dfrac {27}{27}=1$ mol

$A-2,B-1,C-4,D-3$

Explain:
Atom is neutral with respect to the electrical charge.

In an atom, the number of protons and electrons are equal. Since the positive and negative charges are equal, an atom is neutral with respect to the electrical charge.

Define an Atom, and give suitable examples.

Atoms are defined as “the basic building blocks of matter” or the smallest particle of an element, which may or may not have an independent existence but always takes place in a chemical reaction is called an atom.

Atoms consist of three fundamental types of sub-particles i.e protons, electrons, and neutrons. Neutrons and protons have approximately the same mass and in contrast to this, the mass of an electron is negligible. A proton carries a positive charge, a neutron has no charge and an electron is negatively charged.

Example: Hydrogen

$(H)$ , Oxygen

$(O)$ , etc.

The symbol 'u' used for expressing atomic and molecular masses represents _ scale based on _ isotope.

The symbol 'u' used for expressing atomic and molecular masses represents the atomic mass scale based on the C-12 isotope of carbon. It is a unified scale.

1 amu defines precisely the 1/12 the mass of an atom of carbon-12.

What are the main postulates of Dalton's Atomic Theory? What were its limitations? How has the theory been modified?

1940695_1866125_ans_749b74f6db464d7c8c11fbf6095d7df9.jpg

What are the postulates of Dalton's Atomic theory?

All matter is made up of tiny, indivisible particles called atoms.
All atoms of a specific element are identical in mass, size, and other properties. However, atoms of different element exhibit different properties and vary in mass and size.
Atoms can neither be created nor destroyed. Furthermore, atoms cannot be divided into smaller particles.
Atoms of different elements can combine with each other in fixed whole-number ratios in order to form compounds.
Atoms can be rearranged, combined, or separated in chemical reactions

Define 'Law of constant composition or definite Proportions'. Explain with a suitable example.

This law states that "in a chemical substance, elements are always present in definite proportions by mass".

For example:

In water

$H_2O$ , hydrogen and oxygen are always present in the ratio of 1:8 by mass, whatever the method or source from which water is obtained.

Massof 2 hydrogen atom=2

Mass of one oxygen atom=16

Hence, ratio of hydrogen and oxygen by mass=

$2:16$ =

$1:8$

Define Molecular mass, and give suitable examples.

The molecular mass of a substance is the sum of atomic masses of all the atoms in a molecule of the substance. It is used for those substances whose constituent particles are molecules.

For example:

The moleculear mass of

$CO_2=(1× \text{atomic mass of carbon}) + (2 × \text{atomic mass of oxygen})$

$⇒12+2×16$

$⇒12+32=44u$

Define Molecule. Give suitable examples.

A group of atoms that combine together and form a single entity. Ex-

$H_2O$ where 2 atom of H and 1 of oxygen forms a molecule of water.

(a) Calculate the number of oxygen atoms in $0.10$ mole of $Na_2CO_3.10H_2O$ .
(b) If one mole of sulphur weighs $32$ grams, what is the mass (in grams) of $1$ atom of sulphur
(c) Identify the correct formula for ammonium sulphate from the following formula: $(NH_4), (SO_4)_3, (NH_4)_2, (NH_4)_2SO_4$

(a)

$\text{Step 1: Finding the number of molecules in 0.10 mole of }$

$Na_2CO_3.10H_2O$

Since,

$1\ mole=6.022\times 10^{23} molecules$

Therefore,

$1\ mole\ of\ Na_2CO_3.10H_2O=6.022\times 10^{23} molecules\ of\ Na_2CO_3.10H_2O$

So,

$0.1\ mole\ of\ Na_2CO_3.10H_2O\ will\ have=6.022\times 10^{23}\times 0.1\ molecules\ of\ Na_2CO_3.10H_2O$

$\text{Step 2: Calculation of number of oxygen atoms present in 0.10 mole of }$

$Na_2CO_3.10H_2O$

$1\ mole$

$Na_2CO_3.10H_2O$ contains 13 oxygen atoms.

Total number of oxygen atoms in 1 mole

$(6.022\times 10^{23} atoms)$ of

$Na_2CO_3.10H_2O$

$=6.022\times 10^{23}\times 13$

Therefore,

Total number of oxygen atoms in

$0.10$ mole of

$Na_2CO_3.10H_2O$

$=6.022\times 10^{23}\times 13\times 0.1$

$=7.82\times 10^{23} atoms$

Hence,

$0.10\ mole$ of

$Na_2CO_3.10H_2O$ contains

$=7.82\times 10^{23} oxygen\ atoms$

(b)

$1$ mole of sulphur contains

$6.022\times 10^{23}$ atoms

Now, mass of

$1$ mole

$(6.022\times 10^{23} atoms)$ of sulphur

$=32g$

Therefore, mass of

$1$ atom of sulphur

$=\dfrac{32}{6.022\times 10^{23}} g$

$=5.31\times 10^{-23}$ g

Hence, the mass of

$1$ atom of sulphur is

$5.31\times 10^{-23}$ g

(c)

$(NH_4)_2SO_4$ is the chemical formula of ammonium sulphate.

$\bullet$ Ammonium sulphate is made by treating ammonia, often as a by-product from coke oven, with sulphuric acid.

$2NH_3 + H_2SO_4\rightarrow (NH_4)_2SO_4$

The diameter of the zinc atom is $2.6\overset{o}A$ . Calculate (a) radius of zinc atom in pm and (b) a number of atoms present in a length of $\text{1.6 cm}$ if the zinc atoms are arranged side by side lengthwise.

(i)The diameter of zinc atom is

$\displaystyle 2.6 \overset{o}{ A} = 2.6 \times 10^{-10}m$ .
The radius of Zn atom is

$\displaystyle \frac {2.6 \times 10^{-10} }{2} = 1.3 \times 10^{-10}m =130 \times 10^{-12}m =130\ pm$ .
(ii) The number of Zn atoms present on 1.6 cm of length are

$\displaystyle \frac {1.6}{2.6 \times 10^{-8} }=6.154 \times 10^{7}$ .

2.8 g of calcium oxide ( $CaO$ ) prepared by heating limestone was found to contain 0.8 g of oxygen. When one gram of oxygen was treated with calcium, 3.5 g of calcium oxide was obtained. Show that the results illustrate the law of definite proportions.

Law of definite proportion states that a given compound always contains exactly the same proportion of elements by weight.

$\underline{\text{ Case 1:}}$

$Ca\\2.8g$

$O\\0.8g$

$\Longrightarrow 1g$ of

$O$ reacts with

$=\cfrac{2.8}{0.8}=3.5g$ of

$CaO$

$\underline{\text{Case 2:}}$

$3.5g$

$1g\Longrightarrow 1g$ of

$O$ reacts qith

$3.5g$ of

$CaO.$

Ratio of

$O$ reacting with

$Ca$ to form

$CaO$ is

$1:1$ .

Hence results illustrate the law of definite proportion.

Calculate the effective nuclear charge at the periphery of nitrogen atom when an extra electron is added in the formation of anion. Also calculate the effective nuclear charge of N atom.

1350675_1139782_ans_7329eadefe374f3cadf17918ac9c485c.jpg

A gaseous mixture of ${H}_{2}$ and ${N}_{2}O$ gas contains $66$ mass $\%$ of ${N}_{2}O$ . What is the average molecular mass of the mixture?

Mass of

${ H }_{ 2 } =2$

Mass of

${ N }_{ 2 }O$ =

$(14\times 2)+16$ =

$44$

${ N }_{ 2 }O$ contains 66% of mass then the remaining 34% will be

${ H }_{ 2 }$

Then the average molecular mass will be

$=\dfrac { (44\times 66)+(34\times 2) }{ 100 } \\ \quad$

$\dfrac { 2972 }{ 100 } \\ \quad$ =

$\quad 29.72$

Cinnabar(HgS) is a prominent ore of mercury. How many grams of mercury are present in 225g of pure HgS? Molar mass of Hg and S are 200.6 g $mol^{ - 1}$ and 32 g $mol^{ - 1}$ respectively.

1986836_1230273_ans_271cd71f9b3c4ca1aff5729328500d2d.jpg

A sample of vitamin $C$ is known to contain $2.58\times 10^{24}$ oxygen atom. How many moles of oxygen atoms are present in the sample?

1871119_1430170_ans_3d89113263c149d6b88065f00894564a.jpg

A sample of ethane $(C_2H_6)$ gas has the same mass as $1.5\times 10^{20}$ molecules of methane $(CH_4)$ . How many $C_2H_6$ molecules dose the sample of gas contains

1873819_1343129_ans_c8b8f77621e94f92a20069853f030940.jpeg

Classify each of the following on the basis of their atomicity :

$C_2H_6$

1987783_1791865_ans_cb2a6cb2328d41f2ae7225d1a04f3456.jpg

Classify each of the following on the basis of their atomicity:
$H_2O_2$

1904337_1791868_ans_7d0c35a3078a4b27903522876fbe825e.jpg

A vessel of volume $3.0$ l contains ideal gas at the temperature $0^o$ C. After a portion of the gas has been let out, the pressure in the vessel decreases by $0.8$ atm(the temperature remains constant). The gas density under the normal condition( $0^o$ C and $1$ atm) is $1.25$ g/l. The mass of released gas in(g) is?

Vol of gas

$=$ vol of container

$=3$ lit (constant)

$d$ (at

$0^o$ C and

$1$ atm)

$=1.25$ g/lit

$T=0^o$ c,

$p=1$ atm, wt of gas

$=1.25\times 3$

$=3.75$ g

Now, some gas go out, say n gm

$\therefore$ mass of gas in container

$=(3.75-n)$ gm

T=constant

P decreases by

$0.8$ atm

$\therefore P_{new}=0.2$ atm

Now,

$P\propto n$

$\therefore \dfrac{p_1}{p_2}=\dfrac{wt_1}{wt_2}\Rightarrow \dfrac{1}{0.2}=\dfrac{3.75}{3.75-n}$

$3.75-n=3.75\times 0.2=0.75$ gm

$\therefore n=3$ gm gas goes out.

thus, answer is

$3$ .

Benzoic acid is monobasic and when $1.22g$ of its pure sample after dissolution in distilled water is titrated against a base it uses $50mL$ of $0.2M$ $NaOH$ . Calculate the molar mass of benzoic acid.

Meq of benzoic acid

$=\cfrac { w }{ E } \times 1000=\cfrac { 1.22 }{ M/1 } \times 1000=\cfrac { 1220 }{ M }$

Meq of

$NaOH=50\times 0.2=10$

At equivalence point Meq. of Benzoic acid=Meq. of

$NaOH$

$\therefore \cfrac { 1220 }{ M } =10\Rightarrow M=122$

Discuss the principle and the method of calculation of the following :
Volumetric method for the determination of molecular mass of acids and bases.

Volumetric analysis is a quantitative analytical method which is used widely. As the name suggests, this method involves measurement of the volume of a solution whose concentration is known and applied to determine the concentration of the analyte

In volumetric analysis the weight of a reagent necessary to complete a chemical reaction is ascertained by determining the volume of a solution containing a known concentration of the reagent required to react stoichiometrically with the element. From this volume the percentage of the element can be calculated.

Procedure for Volumetric Analysis

A typical titration starts with a beaker or flask containing a precise volume of the analyte and small amount of indicator placed underneath a calibrated burette or pipette containing the titrant.
The solution that needs to be analysed needs to have an accurate weighed in the sample of +/- 0.0001g of the material to be analysed.
Choosing the right kind of material to be analysed is also very important, as choosing the wrong type of titrant will give us the wrong results. A substance that reacts rapidly and completely to produce a complete solution is chosen.
Small quantities of titrant are added to the analyte and indicator till the indicator changes colour in reaction to the titrant saturation threshold reflects the arrival at the endpoint of the titration.
The titration has to be continued up until the reaction is complete and the amount of reactant added is exactly the amount that is needed to complete the reaction.
Another important step is in measuring the right volume of the standard solution since molarity is a standard metric to calculate the number of moles present in a solution.
Based on the desired endpoint, single drops or less than a drop of the titrant makes a difference between a permanent and temporary change in the indicator.
If the reagent or reactant that we use is to be made into a standard solution then we can weigh and dissolve the reagent into a solution, so that it is in a definitive volume within a volumetric flask

Name of Radical	Molecular representation
Sulphate
	$N^{3-}$
Nitrate
Bisulphite
	$CO_{3}^{2-}$
	$OH^{-}$
Phosphate

Atomic Number	Mass Number	Number of Nutrons	Number of Protons	Number of Electrons	Name of the Atomic Species
9	-	10	-	-	-
16	32	-	-	-	Sulphur
-	24	-	12	-	-
-	2	-	1	-	-
-	1	0	1	0	-

Atoms And Molecules - Class 9 Chemistry - Extra Questions

Define ion.

Define Symbol.

Why does OH−OH^{ - } is an ion?

According to Dalton,______is the smallest indivisible particle of the matter.

Complete the statements given below by filling in the blank with the correct word/s.The basic unit of an element is an ________.

Fill in the blanks:The molecules are made up of _________.

__________ are made up of same kind of atoms.

An ion with positive charge is called

Complete the statements given below by filling in the blank with the correct word/s.Atom contains ______, with positively charged _______.

Select the correct answer from the choice in brackets.The symbol of — Mercury [Mg / Hg / Ag]

If an atom loses an electron, it acquires a ____ charge.

If an atom gains an electron, it is called _____.

State Dalton's atomic theory.

A charged particle is called an _____.

An ion with negative charge is called as_________.

What does the word 'atom' mean?

What are molecules ?

Name the smallest particle from which matter is made up.

Write the names of elements from the following symbol :Be

Give some more examples of molecules in the respective categories (other than the ones already mentioned).

Write the diatomic molecules from the following H2,Na,C,N2,P4,O2H_2, Na, C, N_2, P_4, O_2

How many moles of hydrogen are present in 1 gm of hydrogen?

Calculate the molecular mass of ethanoic acid , CH3COOH CH_3COOH . (Atomic masses : C=12u;H=1u;O=16u C = 12u ; H = 1u ; O = 16u )

Give the symbols for metals of group 1313.

The weight of 1×1022 1 \times 10^{22} molecules of CuSO4⋅5H2O \mathrm{CuSO}_{4} \cdot 5 \mathrm{H}_{2} \mathrm{O} is _________.

Components in a mixture are present in varying proportions and not in a fixed proportion.

Molecular mass of water is __________ u.

Molar mass of C2H6C_2H_6 (in g/mol) is:(Given atomic weight of C=12C = 12 gm and H=1H = 1gm)

Calculate the molar mass of CO2\displaystyle { CO }_{ 2 }.

Give reason why an atom of an element is electrically neutral?

Calculate the molar mass of CH4\displaystyle { CH }_{ 4 }.

Define atom.

Determine the molecular mass of water.

What is meant by a 'mole' of a substance?

Give an example of the following:A molecule in which central atom is linked to three other atoms.

How many moles are there in 200g200g of NaNa?

Define the following terms:AtomIsotopeAtomic numberAtomic mass number

Write a short note on Avogadro's number .

Which qualities are required in the student to perform such an activity?

What is the difference between cation and anion?

Calculate the number of moles of magnesium present in a magnesium ribbon weighing 12 g. molar atomic mass of magnesium is 24gmole−1 24 g mole^{-1}

How do negative and positive ions form?

What is an atom?

Define an atom.

Calculate the number of hydrogen molecules in 100gm100 \,gm hydrogen.

Calculate the mass (in grams) of - (i) 102310^{23} molecules of CO2CO_{2} gasgas (ii) 2 moles of nitrogen atom (C=12 u, 0= 16 u, N = 14 u)

Fill in the blank.Dalton said that _______ could not be divided.

AB2AB_2 and A2B3A_2B_3 are two compounds of the elements A and B. 0.15 mole of each of these compounds weighs 9.3 and 15.9 g respectively. Find the atomic masses of A and B.

A sample of CaCO3CaCO_3 has Ca (40%), C (12%), O (48%) by mass. If the law of constant proportions is true then find the mass of Ca in 5 g of CaCO3CaCO_3 obtained from another source.

Calculate the number of atoms in 120 g of Ca120\ g\ of\ Ca.

Write formula of - barium sulphate

Ravi prepared a solution of sodium chloride by mixing 5.85 g salt in 1 litre of water. Find\text{Ravi prepared a solution of sodium chloride by mixing 5.85 g salt in 1 litre of water. Find}Molar mass of sodium chloride

The mass of one steel screw is 4.11 g. Find the mass of one mole of the steel screws. Compare this value with the mass of the Earth (5.98×1024)(5.98 \times10^{24}) kg.Which one of the two is heavier and by how many times?

14 g of N2N_2 contains 3.01×10233.01\times 10^{23} nitrogen molecules. If true then enter 1, if false enter 0.

The molar mass of H2OH_2O is:

The molar mass of CO2CO_2 is :

Molar mass of H2SO4H_2SO_4 is:(Given: Atomic mass of H=1,S=32,O=16H=1, S=32, O=16)

If the atomic mass of carbon were to set at 100100 amu, the Avogrado's number will be equal to 5.01×10x5.01\times {10}^x atoms. Find the value of xx.

Select the correct answer from the given choices for the statement given below.A positively charged ion is called [cation / anion]________.

Cations are formed by .............. (loss/ gain) of electrons and anions are formed by ................. (loss/ gain) of electrons.

Identify the odd one out:Mercury, Gold, Brass, Nitrogen

The weight of 1×10221\times 10^{22} molecules of CuSO4⋅5H2OCuSO_4\cdot 5H_2O is:(nearest integer)

What is denoted by the chemical formula S8S_8.

What does the symbol H2H_2 convey?

Calculate the formula mass of compound given below: NaCl (common salt)Given: Atomic masses of Na = 23, Cl = 35.5.

Calculate the molar mass of the following:(i) H2O\displaystyle { H }_{ 2 }O(ii) CO2\displaystyle { CO }_{ 2 }(iii) CH4\displaystyle CH_{ 4 }

What will be the mass of one 12C\displaystyle ^{ 12 }{ C } atom in g ?

Calculate the mass and charge of one mole of electrons. Given: Mass of one electron = 9.10×10−319.10 \times 10^{-31} kgCharge of one electron = 1.602×10−191.602 \times 10^{-19} coulomb

What are the difference between atoms and molecules?

In an experiment 0.2430 gm of magnesium on burning with oxygen yielded 0.4030 gm of magnesium oxide. In another experiment 0.1820 gm of magnesium on burning with oxygen yielded 0.3020 gm of magnesium oxide. Show that the data explain the law of definite proportions.

Mention the difference between N2N_2 and 2N2N?

Convert into mole.(a)(a) 1212 g of oxygen gas(b)(b) 2020 g of water(c)(c) 2222 g of carbon dioxide.

The symbol for element lead is derived from the latin name ---------------.

A symbol of an element is always represented by the first alphabet of an atom of the element.

Match the mass of elements given in column I with the no. of moles given in column II and mark the appropriate choice.

The sodium salt of methyl orange has 7% sodium. What is the minimum molecular weight of the compound?

Which of the following contains the largest number of oxygen atoms? 1.0 g of OO atoms, 1.0 g of O2O_2, 1.0 g of ozone O3O_3?

The order of increasing relative molecular mass of the following gases is, hydrogen, oxygen, carbondioxide, sulphurdioxide, chlorine. Given 8 gram of each gas at STP, which will contain the least number of molecules and which the most?

In two different experiments, the copper oxide was obtained from copper. The ratio of the mass of copper and the mass of oxygen found in the two experiments are the same. They are in the ratio of 4:14: 1. What of you infer about the experiments?

Why does $OH^{ - }$ is an ion?

Complete the statements given below by filling in the blank with the correct word/s.
The basic unit of an element is an ________.

Fill in the blanks:
The molecules are made up of _________.

Complete the statements given below by filling in the blank with the correct word/s.
Atom contains , with positively charged _.

Select the correct answer from the choice in brackets.
The symbol of — Mercury [Mg / Hg / Ag]

Write the names of elements from the following symbol :
Be

Write the diatomic molecules from the following
$H_2, Na, C, N_2, P_4, O_2$

Calculate the molecular mass of ethanoic acid , $CH_3COOH$ .
(Atomic masses : $C = 12u ; H = 1u ; O = 16u$ )

Give the symbols for metals of group $13$ .

The weight of $1 \times 10^{22}$ molecules of $\mathrm{CuSO}_{4} \cdot 5 \mathrm{H}_{2} \mathrm{O}$ is _________.

Molar mass of $C_2H_6$ (in g/mol) is:
(Given atomic weight of $C = 12$ gm and $H = 1$ gm)

Calculate the molar mass of $\displaystyle { CO }_{ 2 }$ .

Calculate the molar mass of $\displaystyle { CH }_{ 4 }$ .

Give an example of the following:

A molecule in which central atom is linked to three other atoms.

How many moles are there in $200g$ of $Na$ ?

Define the following terms:
Atom
Isotope
Atomic number
Atomic mass number

Calculate the number of moles of magnesium present in a magnesium ribbon weighing 12 g. molar atomic mass of magnesium is $24 g mole^{-1}$

Calculate the number of hydrogen molecules in $100 \,gm$ hydrogen.

Calculate the mass (in grams) of -
(i) $10^{23}$ molecules of $CO_{2}$ $gas$
(ii) 2 moles of nitrogen atom
(C=12 u, 0= 16 u, N = 14 u)

Fill in the blank.
Dalton said that _______ could not be divided.

$AB_2$ and $A_2B_3$ are two compounds of the elements A and B. 0.15 mole of each of these compounds weighs 9.3 and 15.9 g respectively. Find the atomic masses of A and B.

A sample of $CaCO_3$ has Ca (40%), C (12%), O (48%) by mass. If the law of constant proportions is true then find the mass of Ca in 5 g of $CaCO_3$ obtained from another source.

Calculate the number of atoms in $120\ g\ of\ Ca$ .

$\text{Ravi prepared a solution of sodium chloride by mixing 5.85 g salt in 1 litre of water. Find}$
Molar mass of sodium chloride

The mass of one steel screw is 4.11 g. Find the mass of one mole of the steel screws. Compare this value with the mass of the Earth $(5.98 \times10^{24})$ kg.Which one of the two is heavier and by how many times?

14 g of $N_2$ contains $3.01\times 10^{23}$ nitrogen molecules. If true then enter 1, if false enter 0.

The molar mass of $H_2O$ is:

The molar mass of $CO_2$ is :

Molar mass of $H_2SO_4$ is:
(Given: Atomic mass of $H=1, S=32, O=16$ )

If the atomic mass of carbon were to set at $100$ amu, the Avogrado's number will be equal to $5.01\times {10}^x$ atoms. Find the value of $x$ .

Select the correct answer from the given choices for the statement given below.
A positively charged ion is called [cation / anion]________.

Identify the odd one out:
Mercury, Gold, Brass, Nitrogen

The weight of $1\times 10^{22}$ molecules of $CuSO_4\cdot 5H_2O$ is:
(nearest integer)

What is denoted by the chemical formula $S_8$ .

What does the symbol $H_2$ convey?

Calculate the formula mass of compound given below:
NaCl (common salt)
Given: Atomic masses of Na = 23, Cl = 35.5.

Calculate the molar mass of the following:

(i) $\displaystyle { H }_{ 2 }O$

(ii) $\displaystyle { CO }_{ 2 }$

(iii) $\displaystyle CH_{ 4 }$

What will be the mass of one $\displaystyle ^{ 12 }{ C }$ atom in g ?

Calculate the mass and charge of one mole of electrons.

Given:
Mass of one electron = $9.10 \times 10^{-31}$ kg
Charge of one electron = $1.602 \times 10^{-19}$ coulomb

Mention the difference between $N_2$ and $2N$ ?

Convert into mole.
$(a)$ $12$ g of oxygen gas
$(b)$ $20$ g of water
$(c)$ $22$ g of carbon dioxide.

Which of the following contains the largest number of oxygen atoms? 1.0 g of $O$ atoms, 1.0 g of $O_2$ , 1.0 g of ozone $O_3$ ?

In two different experiments, the copper oxide was obtained from copper. The ratio of the mass of copper and the mass of oxygen found in the two experiments are the same. They are in the ratio of $4: 1$ . What of you infer about the experiments?

Aluminium reacts with sulphuric acid to form Aluminium sulphate. Answer the following:
What is the charge carried by sulphate ion?

(a) State the postulates of Dalton's atomic theory.
(b) Calculate the concentration in terms of mol/kg when 6.35g of KCl is dissolved in 300g of water. [At mass of K=39u, At mass of Cl=35.5u]

How many water molecules will be there in $3\times 10^{-23}$ g sample of water?

Find the molecular mass (in kilogram) of an ammonia molecule $NH_3$ .

Write the molecular mass of
$HgCO_3,CaCO_3,NaHCO_3,CH_3CH_2OH, Al_2,(SO_4)_2$

Number of molecules present in $4.2$ g $N_2$ is $X\times10^{22}$ . What is the value of $X$ (round off to 1 digit)?

Calculate the number of molecules in a drop of water weighing $0.05$ gm.

Cost of sugar ( ${C}_{12}{H}_{22}{O}_{11}$ ) is Rs. $40$ per kg. Calculate its cost per mole.

What does $^{12}C$ mean in amu?

Name of the ion with symbol $S^{2-}$ _______

Give the answers of the following:
(a) What is the mass of one atom of S?
(b) What is the atomic mass of S?
(c) what is the gram atomic mass of S?

Write the complete symbol for the atoms with given atomic and mass no, $Z=18\ A=40\$ and $A=9$
( $Z=$ Atomic no and $A=$ mass)

Calculate the molecular masses of $H_2, O_2, Cl_2, CO_2, CH_4, C_2H_6, C_3H_6, NH_3, CH_3OH$

$2.8\ g$ of calcium oxide prepared by heating limestone were found to contain $0.8\ g$ of oxygen. When one gram of oxygen was treated with calcium, $3.5\ g$ of calcium oxide was obtained. Show that the results illustrate the law of definite proportions.

Complete the following table of radicals.
Name of Radical Molecular representation
Sulphate
$N^{3-}$
Nitrate
Bisulphite
$CO_{3}^{2-}$
$OH^{-}$
Phosphate

Calculate the molar mass of the following substances
(a) Ethyne $C_2H_2$
(b) Sulphur molecule $S_8$
(c) Phophorous molecule $P_4$
(d) Nitric acid $HNO_3$
(e) Hydrochloride acid $HCl$ .

Find the molecular mass of:
(a) $H_2S$ (b) $HCl$ (c) $NH_3$ (d) $Cl_2$ (e) $CH_3COOH$ (f) $CH_3CHO$

The molecular weight of hydrogen peroxide is $34$ . What is the unit of molecular weight?

Calculate the formula unit mass of $CaSO_{4.}2H_{2}O$ .

What is the symbol of the species with number of electrons equal to $36$ protons equal to $35$ and neutrons equal to $45$ ?

Complete the following table.
Atomic
Number Mass
Number
Number
of
Nutrons
Number
of
Protons Number
of
Electrons
Name of the
Atomic Species
9 - 10 - - -
16 32 - - - Sulphur
- 24 - 12 - -
- 2 - 1 - -
- 1 0 1 0 -

Write the names of the following compounds and deduce their molar masses.
${ N }a_{ 2 }{ SO }_{ 4 },\quad { K }_{ 2 }{ CO }_{ 3 },\quad { CO }_{ 2 },\quad { MgCl }_{ 2 },\quad NaOH,\quad { AlPO }_{ 4 },\quad { NaHCO }_{ 3 }$

An atom of an element is denoted by a "symbol"- Explain the meaning of the term 'symbol'. State a reason for representing the following elements by their symbols.
(a) Hydrogen by 'H'
(b) Helium by 'He'
(c) Copper by 'Cu'

Calculate the molecular masses of $H_{2},O_{2},Cl_{2},CO_{2},CH_{4}, C_{2}H_{6},C_{2}H_{4},NH_{3},CH_{3}OH$ .

What are the ions present in this compound?
$Al_2(SO_4)_3$

Apply the law of constant proportions to calculate the mass of oxygen that will be used up for combustion of 5 g of ${H_2}$ gas.

Calculate the number of moles of the following:
(I) $52g$ of $He$

Water $(H_2O$ ) obtain from any source contains hydrogen and oxygen in the ratio 1:8 by mass. Which Law follows it? Define the Law?

(a) State six postulates of Daltons atomic theory.
(b) A 0.24g Sample of compound of boron and oxygen on analysis was found to contain 0.096g of boron and 0.144g of oxygen. Find the percentage composition of the compound by wight.

Verify by calculation that 5 moles of $CO_2$ and 5 moles of $H_2O$ not have same mass.

Differentiate between ${\text{2H}}$ and ${{\text{H}}_{\text{2}}}$ .

Mass of 1.5 mole of ${ CO }_{ 2\ }$ molecule

Calculate the mass of the following
(i) $0.5$ mole of $N_{2}$ gas (mass from mole of molecule)
(ii) mole of $N$ atoms (mass from mole of atom)
(iii) $3.011\times 10^{23}$ number of N atoms (mass from number)
(iv) $6.022\times 10^{23}$ number of $N_2$ atoms (mass from number)

What is the weight of $3.01 \times 10^{23}$ molecules of ammonia?