Salt of a strong acid and a weak base:

The solution of such a salt is acidic in nature. The cation of the salt which has come from weak base is reactive. It reacts with water to form a weak base and H⁺ ions.

B⁺  +   H₂O  ↔  BOH  +  H⁺

                         Weak base

Consider, for example, NH₄Cl. It ionises in water completely into NH₄ and CF ions. ions react with water to form a weak base (NH₄OH) and H⁺ ions.

    NH⁺₄  +  H₂O   ↔   NH₄OH  +  H⁺

    C(1-x)                   Cx         Cx

Thus, hydrogen ion concentration increases and the solution becomes acidic.

Applying law of mass action,

K_h = [H^x ][NH₄ OH]/[NH₄⁺ ]=(Cx.Cx)/C(1-x) = (x² C)/((1-x))     ...... (i)

where C is the concentration of salt and x the degree of hydrolysis.

Other equilibria which exist in solution are

NH₄OH ↔ NH⁺₄ + OH^-,    K_b = [NH⁺_4][OH^-]/[NH₄Oh]    .... (ii)

H₂O ↔  H⁺ + OH^-,          K_b = [H⁺][H^-]            ..... (iii)

From eqs. (II) and (iii)

K_w/K_b =[H⁺ ][NH⁴ OH]/[NH₄⁺ ] =K_h      .... (iv)

[H⁺] = [H⁺ ][NH₄⁺]/[NH₄OH] = K_w/K_b ×[NH₄⁺ ]/[NH₄ OH]

log [H⁺] = log K_w - log K_b + log[salt]/[base]

-pH = -pK_w + pK_b + log[salt]/[base]

pK_w - pH = pK_b + log[salt]/[base]

pOH = pK_b + log[salt]/[base]