Explanation
The relative acidic strength of formic acid , acetic acid and benzoic acid follows the order,
Formic acid > Benzoic acid > Acetic acid.
The reason behind this trend as follows.The phenyl group of benzoic acid when directly attached to the carboxylic group has an overall electron donating effect. It is however weaker than the +I – effect of any alkyl group . This makes the unsubstituted carboxylic acids stronger than unsubstituted aliphatic carboxylic acids. Further since, formic acid doesnot contain any alkyl group , therefore it is stronger acid than benzoic acid .
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