The greater the number of electron-withdrawing substituents, the greater the dispersal of the negative charge and hence stronger will be the acid. Hence, the order of acidity is
Cl3CCOOH > Cl2CHCOOH > ClCH2COOH > CH3COOH
Strength of carboxylic acids
Carboxylic acids are weak acids because they only partially ionise in solution. Their solutions do not contain many hydrogen ions than a strong acid solution at the same concentration. Acetic acid is ten times weaker than formic acid confirming the electron-donating character of an alkyl group relative to hydrogen.