(i) NH₄Cl is a strong electrolyte hence, dissociates completely. NH₄OH is a weak electrolyte and dissociates to a small extent. Its dissociation is further suppressed by common ion provided by NH₄Cl in the solution. This solution acts as a basic buffer and maintains its pH around 9.25. It resists the change in pH on the addition of a small amount of acid or alkali. This can be explained as below: Upon adding a small amount of HCl to this solution, H⁺ ions of HCl gets neutralised by OH^– ions already present and more of NH₄OH molecules get ionised to compensate for the loss of OH^– ions. Thus, pH practically remains unchanged. Upon adding a small amount of NaOH to this solution, OH ions of NaOH combine with ions already present to form weakly ionised NH4OH. Thus, pH of the solution remains practically unchanged.

(ii) This is because the K_sp of CuS is very low as compared to that of ZnS. Therefore, a low concentration of S^2- ions can precipitate Cu²⁺ ions as CuS, whereas a high concentration of S₂ ions is required to precipitate Zn²⁺ ions as ZnS. Now, in the presence of HCl, dissociation of H₂S is suppressed due to the common ion effect. HCl → H⁺ + Cl^– (strong electrolyte) H₂S → 2H⁺ + S^2- (weak electrolyte)

With decreased S^2- ion concentration only Cu²⁺ is precipitated as CuS.