Salt of a weak acid and a weak base:

Maximum hydrolysis occurs in the case of such a salt as both the cation and anion are reactive and react with water to produce H⁺ and OFT ions. The solution is generally neutral but it can be either slightly acidic or slightly alkaline if both the reactions take place with slightly different rates. Consider, for example, the salt CH₃COONH₄. It gives CH₃COO^- and  ions in solution. Both react with water.

Other equilibria which exist in solution are:

CH₃COOH ↔  CH₃COO^- + H⁺, K_a =  [CH³COO^-][H⁺]/[CH₃COOH]    ..... (i)

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

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

From Eqs. (i), (ii) and (iii),

K_h =  Kw/K_a.K_b = [CH₃COOH][NH₄OH]/[CH₃COO^-][NH⁺₄]                     .... (iv)

Let C be the concentration and h be the degree of hydrolysis

   K_h = h²/(1-h)²

When h is small, (1-h) → 1.

K_h = h²

h = √Kh = √K_w/K_a*K_b

[H⁺] K_a × h

= K_a × √K_w/K_a*K_b

= √K_{w *} K_a/K_b

-log [H⁺] = -1/2log K_a - 1/2log K_w + 1/2log K_b

pH = 1/2pK_a +  1/2pK_w - 1/2pK_b

= 7 +  1/2pK_a - 1/2pK_b

When pK_a = pK_b, pH = 7, i.e., solution will be neutral in nature.

When pK_a > pK_b. The solution will be alkaline as the acid will be slightly weaker than base and pH value will be more than 7. In case pK_a < pK_b, the solution will be acidic as the acid is relatively stronger than base and pH will be less than 7.

Salt of a strong acid and a strong base

Such a salt, say NaCl, does not undergo hydrolysis as both the ions are not reactive. The solution is thus, neutral in nature.