Question

Explain the effect of common ion on the dissociation of weak base.

Answer 1

(1) Consider the dissociation or ionisation of a weak base, NH₄OH in its dilute solution.

NH₄OH_(aq) ⇌ NH⁺_4(aq) + OH⁻_(aq)

The dissociation constant K_b for NH₄OH will be,

K_b = \(\frac{[NH^+_4]\times [OH^-]}{[NH_4OH]}\)

(2) If a strong electrolyte like salt NH₄Cl is added to the solution of NH₄OH, then it gives common ion \(NH^+_4\).

NH₄Cl →\(NH^+_4\) + Cl^-

(3) Due to common ion \(NH^+_4\), overall concentration of \(NH^+_4\) is increased, which increases the ratio [\(NH^+_4\)] x \(\frac{[OH^-]}{[NH_4OH]}\),

In order to keep this ratio constant, the equilibrium is shifted to the left hand side which satisfies Le Chatelier’s principle.

(4) Thus the ionisation of a weak base is suppressed by a common ion.