Q. Arrange in increasing acidic strength
The relative acidic strength of formic acid, acetic acid and benzoic acid follows the sequence:
Formic acid > Benzoic acid > Acetic acid
Let us now understand the reason behind this trend.
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 acids than unsubstituted aliphatic carboxylic acids.
Further since, formic acid doesn't contain any alkyl group, therefore it is a stronger acid than benzoic acid.