Question

A solution has 0.1 M Mg²⁺ and 0.05 M NH₃. Calculate the concentration of NH₄Cl required to prevent the formation of Mg(OH)₂ in solution. K_sp[Mg(OH)2] =18.0 × 10^–12 and ionisation constant of NH₃ is 1.8 × 10^–5.

Answer 1

The minimum [OH^–] at which there will be no precipitation of Mg(OH)₂ can be obtained by K_sp = [Mg²⁺] [OH^–]² 

⇒ 18.0 × 10^–12 = (0.1) × [OH^–]² 

∴ [OH^–] = 1.34 × 10^–5 M

Thus, solution having [OH^–] = 1.34 × 10^–5 M will not show precipitation of Mg(OH)₂ in 0.1 M Mg²⁺. These hydroxyl ions are to be derived by basic buffer of NH₄Cl and NH₄OH.

In presence of [NH₄Cl], all the NH₄⁺ ions provided by NH₄Cl as due to common ion effect, dissociation of NH₄OH will be suppressed.