Question

What is the minimum pH required to prevent the precipitation of ZnS in a solution that is 0.01 M ZnC and saturated with 0.10 M H₂S ?

[Given K_sp = 10^–21, $K_{a_{1}}$ × $K_{a_{2}}$= 10^–20]

• A. 0
• B. 1
• C. 2
• D. 4

Answer 1

Answer: (B)

1

Answer 2

K_sp = [Zn²⁺] [S^2–]

[S^2–] = $\frac{10^{–21}}{0.01}$= 10^–19

for $K_{a_{1}}$. $K_{a_{2}}$ = $\frac{\lbrack H^{+}\rbrack^{2}\lbrack S^{2–}\rbrack}{\lbrack H_{2}S\rbrack}$

10^–20 = $\frac{\lbrack H^{+}\rbrack^{2} \times 10^{–19}}{0.1}$̃ [H⁺] = 0.1

or pH = 1