Carboxylic Acids as Weak Acids

Carboxylic acids are weak acids: they only partially dissociate in water, releasing a small proportion of their H⁺ ions. For ethanoic acid:CH₃COOH ⇌ CH₃COO⁻ + H⁺The position of equilibrium lies well to the left, so most molecules stay undissociated. The anion formed is a carboxylate ion; CH₃COO⁻ is the ethanoate ion.The strength of a weak acid is measured by its acid dissociation constant:K_a = [CH₃COO⁻][H⁺] ÷ [CH₃COOH]Carboxylic acids are much weaker than strong acids such as HCl, but they are still acidic enough to react with metals, bases and carbonates to form salts (named -oates, e.g. sodium ethanoate).Worked example: find the pH of 0.100 mol dm⁻³ ethanoic acid, K_a = 1.74 × 10⁻⁵ mol dm⁻³. Assume [H⁺] = [CH₃COO⁻] and that the acid concentration hardly changes, so[H⁺] = √(K_a × [HA]) = √(1.74 × 10⁻⁵ × 0.100) = 1.32 × 10⁻³ mol dm⁻³pH = −log(1.32 × 10⁻³) = 2.88. A strong acid of the same concentration would have pH 1.00, which shows how little the carboxylic acid dissociates.