(i) Phenoxide ion has non-equivalent resonance structures in which the negative charge is at the less electronegative carbon atom whereas in case of carboxylate ion both the resonating structures are equivalent.
(ii) The negative charge is delocalised over two electronegative oxygen atoms in carboxylate ion whereas in phenoxide ion the negative charge less effectively delocalises over one oxygen atom and less electronegative carbon atoms. So the carboxylate ion is more resonance stabilised than phenoxide ion. Thus, the release of proton from carboxylic acid is much easier than from phenol. Hence, carboxylic acid is a stronger acid than phenol.
