Question

The best way to ensure complete precipitation of a metal sulphide $MS$ from saturated $H_{2}S\left(aq\right)$ with a metal ion $M^{2+}$ is by :

• A. adding an acid
• B. increasing $\left[H_{2}S\right]$ in solution
• C. raising the pH
• D. heating the solution

Answer 1

Answer: (C)

raising the pH

$H_{2}S\rightleftharpoons 2H^{+}+S^{2-}$
$S^{2-}+M^{2+}\rightleftharpoons MS$
When pH is increased, the hydrogen ion concentration decrease.
To nullify the effect of a decrease in the hydrogen ion concentration, the dissociation equilibrium of hydrogen sulphide shifts to right so that more and more hydrogen ions are obtained. This also increases the concentration of sulphide ions.
The increase in the concentration of sulphide ions shifts the equilibrium for the formation of metal sulphide towards the right.
This ensures complete precipitation of metal sulphide