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.

            salt-of-a-weak-acid-and-a-weak-base

Other equilibria which exist in solution are:

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

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

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

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

Kh =  Kw/Ka.Kb = [CH3COOH][NH4OH]/[CH3COO-][NH+4]                     .... (iv)

 

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

   Kh = h2/(1-h)2

When h is small, (1-h) → 1.

Kh = h2

h = √Kh = √Kw/Ka*Kb

[H+] Ka × h

= Ka × √Kw/Ka*Kb

= √Kw * Ka/Kb

-log [H+] = -1/2log Ka - 1/2log Kw + 1/2log Kb

pH = 1/2pKa +  1/2pKw - 1/2pKb

= 7 +  1/2pKa - 1/2pKb

When pKa = pKb, pH = 7, i.e., solution will be neutral in nature.

When pKa > pKb. The solution will be alkaline as the acid will be slightly weaker than base and pH value will be more than 7. In case pKa < pKb, 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.

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