Question

Assuming the complete dissociation of HCl and lead salt, calculate how much HCl is added to $0.001$m lead salt solution to just precipitate when saturated with $H_{2}S$. The concentration of $H_{2}S$. The concentration of $H_{2}S$ is $0.1$M

Answer 1

$\left[P{b}^{+2}\right]\left[S^{-2}\right]=K_{sp}=3.4\times {10}^{-28}$

Saturated means $\left[Pbs\right]=\left[P{b}^{+2}\right]=0.001M$

$\left[s^{-2}\right]=3.4\times {10}^{-25}$

$K_a=1.1\times {10}^{-23}=\frac{\left[H^{+}{]}^2\right[S^{-2}]}{\left[H_{2}S\right]}$

$\Rightarrow [H^{+}{]}^2=\frac{1.11}{3.4\times {10}^2\times 0.1}$

$\Rightarrow \left[H^{+}\right]=1.8$

$\therefore 1.8M$ $HCl$ is added to prevent precipitation.