Isoelectronic pairs are those which have same numbers of electrons in molecules:

(A) $CO_3^{2 - }$ and $NO_3^ -$

$C = 6{e^ - },\,\,O = 8{e^ - }$ and $2{e^- }$ from ve charge.

So, $CO_3^{2 - } \Rightarrow 6 + \left({3 \times 8} \right) + 2 = 8 + 24 = 32{e^ - }$

And $NO_3^ -$       $N = 7{e^ - },\,\,\,O = 8{e^ - }$

$NO_3^ - \left( {{\text{total }}{e^ -}} \right) = 7 + \left( {3 \times 8} \right) + 1 = 32{e^ - }$

Both $CO_3^{2 - }$ and $NO_3^ -$ have 32 electrons each and

Hence they have same number of electrons. 

So isoelectronic

(B) $PO_4^{3 - }\, \Rightarrow \,\,P =15{e^ - }$

$PO_4^{3 - }\, \Rightarrow 15 + \left({8 \times 9} \right) + 3 = 15 + 3 + 32 = 50{e^ - }$ And $SO_4^{2 - } \Rightarrow S\,{\text{has }}16{e^ - }$

$SO_4^{2 - } \Rightarrow 16 + \left( {4\times 8} \right) + 2 = 16 + 2 + 32 = 50{e^ - }$

$PO_4^{3 - }$ and $SO_4^{2 - }$ isoelectronic

(C) $CO$ and ${N_2}$    

Here $C = Z\left( {{\text{atomic number}}} \right)$

$O = 8$, $N = 7$

$6 + 8$ and $7 + 7$

$14{e^ - } = 14{e^ - }$ isoelectronic.

Here: Mass of ${O^ - } = 16\,g/mol$ and

charge ${O^ - } = {\text{one unit}}$

$= 1.6 \times {10^{ - 19}}C$

So,

$e/m = \dfrac{{{\text{Charge}}}}{{{\text{Mass}}\,{\text{ratio}}}}$

$= \dfrac{{1.6 \times {{10}^{ -19}}C}}{{16\,g/mol}}$

$= \dfrac{{1.6 \times {{10}^{ - 19}}\times C \times mol}}{{16\,g}}$

$= \dfrac{{0.1 \times {{10}^{ - 19}}\times C \times NA}}{g}\,\,\,\,\,\,\,\,\,\boxed{mol = NA}$

$e/m = {10^{ - 20}}\dfrac{C}{g}$

Here: Given mass of $C{O_2} =8.8\,{\text{gram}}$

Molar mass of $C{O_2} =44\,{\text{gram}}$

Now, number of moles of $C{O_2} = \dfrac{{8.8}}{{44}}= 0.2\,{\text{moles}}$

$n = 2$

Using $PV = nRT$

$V = \dfrac{{nRT}}{P} =$ volume occupied.

$n = 0.2,\,\,R = \dfrac{{0.0821\,\,L -atm}}{{K\,mol}}$

$T = 31{\,^o}C = 31 + 273 = 304K$

$P = 1\,{\text{atm}}$

${\text{Volume}} = \dfrac{{0.2\,mol\times 0.0821 \times L - atm \times 304K}}{{1\,atm \times K\,mol}}$

$= 0.2 \times 0.0821 \times 304L$

$= 4.99$

Approx. $5$ Liters

Here $A = 78{\,^o}C,\,\,\,B =213{\,^o}C,\,\,\,C = 100{\,^o}C,\,\,D = {\,^o}C,154{\,^o}C$ have melting points.

As stronger the faces of attraction between the particles more will be melting point and vice-versa.

Hence,

Order of their inter-particle forces of attraction is $B > D > C > A$ maximum melting point, so maximum inter particles forces of attraction.

Sponge is a solid state of matter yet it is compressible in nature. This is because of space/Interparticle space between the particles of sponge.

So, the sponge can be compressed. Yet it is solid in nature because there is a force of attraction between the particles of sponge.

Part (a) on hot sunny day, we sprinkle water on roof or open ground because the water can absorb the heat and get converted into vapours.

Since water absorb heat so temperature of surrounding fell down and we get cooling.

Part (b) As perspiration takes place it will gives sweat, sweat will absorb the heat from our body it makes your skin feel cooler when its wet.

Hence in evaporation of sweat will cause absorption of heat making your body cold.

Part (c) Ice has latent heat, it will absorb the heat from burnt skin. Ice get converted into liquid. So it will absorb heat and gives relaxation from burning.