Question

What is the minimum $pH$ required to prevent the precipitation of $ZnS$ in a solution that is $0.01M$ $ZnCl_{2}$ and saturated with $0.10M$ $H_{2}S$ ?
[Given $K_{s p}=10^{- 21},K_{a_{1}}\times K_{a_{2}}=10^{- 20}$ ]

• A. 0
• B. 1
• C. 2
• D. 4

Answer 1

Answer: (B)

$K_{s p}=\left[Z n^{2 +}\right]\left[S^{2 -}\right]K_{e q}=\frac{\left[\right. H^{+} \left]\right.^{2} \left[S^{2 -}\right]}{\left[H_{2} S\right]}$
$10^{- 21}=0.01\left[S^{2 -}\right]10^{- 20}=\frac{\left[\right. H^{+} \left]\right.^{2} 1 0^{- 19}}{\left[0.1\right]}$
$S^{2 -}=10^{- 19}H^{+}=10^{- 1}$