Answer:-   The relative values of the solubility product (Ksp) of sulfides for cations in Group II and Group IIIB are crucial in understanding why they are precipitated under different conditions.

1. Group II Cations (e.g., Pb²⁺, Cu²⁺):
   • These cations have relatively low solubility products for their sulfides.
   • When H₂S gas is passed into an acidic solution containing HCl, it creates an acidic environment with excess H+ ions.
   • The presence of H+ ions suppresses the dissociation of H₂S, reducing the concentration of S^2-ions available in the solution.
   • With the reduced S^2- ion concentration, the solubility product (Ksp) for the sulfides of Group II cations is exceeded, causing them to precipitate as insoluble sulfides.
2. Group IIIB Cations (e.g., Al³⁺, Fe³⁺):
   • These cations have higher solubility products for their sulfides compared to Group II cations.
   • To precipitate the sulfides of Group IIIB cations, an ammoniacal (alkaline) solution of H₂S is used.
   • In this alkaline environment, there are fewer H+ ions present.
   • The reduced H+ ion concentration allows for greater dissociation of H₂S, resulting in a higher concentration of S^2- ions in solution.
   • The increased S^2- ion concentration is necessary to exceed the higher Ksp values for the sulfides of Group IIIB cations, leading to their precipitation.

In summary, the relative solubility products of the sulfides of these cations determine the conditions under which they can be selectively precipitated. Group II cations with lower Ksp values require an acidic environment with H₂S and HCl, while Group IIIB cations with higher Ksp values precipitate in an alkaline solution of H₂S.