Question

What is the minimum pH required to prevent the precipitation of ZnS in a solution which is 0.01 M $ZnCl_{2}$ and saturated with 0.1 M $H_{2}S?$
$\left(\text{K}\right)_{\text{sp}} \, \text{of} \, \left(\text{ZnS}\right) \, \text{=} \, \left(\text{10}\right)^{- \text{21}} \text{,} \, \left(\text{K}\right)_{\left(\text{a}\right)_{\text{1}}} \, \text{\times } \, \left(\text{K}\right)_{\left(\text{a}\right)_{\text{2}}} \left(\left(\text{H}\right)_{\text{2}} \text{S}\right) \, \text{=} \, \left(\text{10}\right)^{- \text{20}}$

• A. 4
• B. 3
• C. 2
• D. 1

Answer 1

Answer: (D)

$\text{K}_{\text{sp}} \, \text{=} \, \text{[Zn}^{\text{2+}}\text{]} \, \text{\times } \, \text{[S}^{\text{2} -}\text{]}$
$\left[\right.S^{2 -}\left]\right.=\frac{K_{s p} Z n S}{\left[\right. Z n^{2 +} \left]\right.}=\frac{1 0^{- 21}}{0.01}=10^{- 19}$
$K_{H_{2} S}=\frac{\left[\right. H^{+} \left]\right.^{2 -} \left[\right. S^{2 -} \left]\right.}{\left[\right. H_{2} S \left]\right.}$
$10^{- 20} = \frac{\left[\right. \text{H}^{+} \left]\right.^{2} \times \left[\right. 10^{- 19} \left]\right.}{0.1}$
$\left[\right.H^{+}\left]\right.=0.1$
$\text{pH} \, \text{=} \, \text{1}$