Why is only As^{3+} precipitated as As_2S_3 w | vedclass

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Question

(A) In an acidic medium,the concentration of $S^{2-}$ ions is very low due to the common ion effect of $H^+$ ions from the strong acid,which suppresse

Vedclass · Accepted Answer

The solubility of $As_2S_3$ is lower than the solubility of $ZnS$.

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(A) In an acidic medium,the concentration of $S^{2-}$ ions is very low due to the common ion effect of $H^+$ ions from the strong acid,which suppresses the dissociation of $H_2S$ $(H_2S \rightleftharpoons 2H^+ + S^{2-})$. The solubility product $(K_{sp})$ of $As_2S_3$ is extremely low compared to that of $ZnS$. Even with the low concentration of $S^{2-}$ ions,the ionic product of $As^{3+}$ and $S^{2-}$ exceeds the $K_{sp}$ of $As_2S_3$,leading to its precipitation. However,for $ZnS$,the ionic product remains lower than its $K_{sp}$ due to its relatively higher solubility,so $ZnS$ does not precipitate.

Why is only $As^{3+}$ precipitated as $As_2S_3$ when $H_2S$ is passed through an acidic solution containing $As^{3+}$ and $Zn^{2+}$,while $Zn^{2+}$ is not precipitated as $ZnS$?

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