Even when acetate ion is stabilized due to resonance why is it a weaker acid than formic acid. It is true that acetic acid has +I effect due to methyl group present on it but resonance is a stronger effect than inductive effect. Then why is it so?
In comparing acidic strength between molecules firstly make its conjugate base , by -H+. Remember the fact Stability of conjugate base is directly proportional to acidic strength of its corresponding molecule. Also while comparing acidic strength check factors that are uncommon to all molecules and increase stability (check electronic effect and also steric hindrance should be kept in mind) .electron withdrawing group increases acidic strength and electron donating group decreases acidic strength. Here, after making conjugate base we have, formate ion and acetate ion. Uncommon part is CH3 bonded and H bonded to acetate and formate.Resonance in C double bonded to O bonded to O- is common to both. Ch3 is electron donating thus decreases acidic strength. H bonded to C is reference and 0 inductive effect is taken for it.Thus, Acetic acid is weaker than Formic acid.