Question

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

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 

[Zn²⁺] [S^2–] < K_sp(ZnS) = 1 × 10^–21 

(ionic product) 

or (0.1) [S^2–] < 1 × 10^–21 

or [S^2–] < 1 × 10^–20 

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

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