Question

Cu is precipitated as CuS while  Zn is not precipitated when H₂S is passed through an acidic solution of Cu(NO₃ )₂  and Zn(NO₃)₂ respectively.

Answer 1

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.