Question

Explain the hydrolysis of the salt of weak acid and weak base.

Answer 1

(1) Consider a salt BA of weak acid (HA) and weak base (BOH). 

(2) In aqueous solution it undergoes hydrolysis as follows :

(3) The nature of the solution will depend upon relative strength of weak acid and weak base, hence will depend upon their dissociation constants K_a and K_b.

(i) A salt of weak acid and weak base for which K_a > K_b :

Consider hydrolysis of NH₄F.

Since K_a(7.2 × 10^-4) for HF is greater than K_b (1.8 × 10^-5) for NH₄OH, the acid dissociates partially more than the base, hence, [H₃O⁺] > [OH^-] and the solution reacts acidic after hydrolysis.

(ii) A salt of weak acid and weak base for which K_a < K_b :

Consider hydrolysis of NH₄CN.

Since K_a(4 × 10^-10) for HF is less than K_b(1.8 × 10^-5) for NH₄OH, the base dissociates more than acid and hence [H₃O⁺] < [OH^-] and the solution reacts basic after hydrolysis.

(iii) A salt of weak acid and weak base for which K_a = K_b :

Consider hydrolysis of CH₃COONH₄.

Since K_a = K_b, the weak acid CH₃COOH and weak base NH₄OH dissociate to the same extent, hence, [H₃O⁺] = [OH^-] and the solution reacts neutral after hydrolysis.