Question

If [Cd²⁺] = [Zn²⁺] = 0.1 M initially, what [H⁺] must be maintained in a saturated H₂S (0.1 M) to precipitate CdS but not ZnS ? 

K_sp (CdS) = 8 × 10^–27 

K_sp (ZnS) = 1 × 10^–21 

K_a (H₂S) = 1.1 × 10^–21 

Answer 1

In order to prevent precipitation of ZnS,

This is the maximum value of [S^2–] before ZnS will precipitate. Let [H⁺] to maintain this [S^2–] be x.

∴ No ZnS will precipitate at a concentration of H⁺ greater than 0.1 M.