In acetic acid, the methyl group is an electron donating group. The acetate ion formed gets destabilized due to the electron releasing effect of methyl group (+1 effect) which is higher than that of H-atom in the corresponding formic acid. As a result, acetic acid dissociates to a lesser extent. Thus decreasing the acidity of acetic acid.
Formic acid having lower pK value than acetic acid. Hence, formic acid is a stronger acid than acetic acid.
