Question

Different solutions containing $MnS,FeS,ZnSandHgS$ have the following concentrations :
$0.01M$ concentration of each $M{n}^{2+},F{e}^{2+},Z{n}^{2+}$ and $H{g}^{2+}$
$\left[S^{2-}\right]={10}^{-8}M$ each.

Given $K_{sp}$ values of $MnS,FeS,ZnSandHgS$ are ${10}^{-14},{10}^{-19},{10}^{-25}$ and ${10}^{-54}$ respectively.
Which of the following statements are correct?

• A. Every salt precipitates at the given concentrations.
• B. Only $MnS$ and $FeS$ precipitates at the given concentrations.
• C. $MnS$ precipitates the earliest.
• D. $HgS$ precipitates the earliest.

Answer 1

Answer: (A), (D)

$HgS$ precipitates the earliest.

Ionic product $\left(Q_{sp}\right)$ of the all the salts $=0.01\times {10}^{-8}={10}^{-10}.$.
The salts can precipitate when the ionic product at given concentration is greater than the solubility product.
Since $\frac{Q_{sp}}{K_{sp}}>1$ for all.
So , every salt precipitates at given concentrations.
Since, the given salts are in their stoichiometric ratios ,they will precipitate in the order of their $K_{sp}$ value.

$HgS$ has the lowest $K_{sp}$ value $\left({10}^{-54}\right)$ so it precipitates the earliest., followed by $ZnS,FeS$ , and $MnS$ precipitates at last, since it has the highest $K_{sp}$ value.