Qualitative Analysis - Result Question 56
####59. During the qualitative analysis of a mixture containing $Cu^{2+}$ and $Zn^{2+}$ ions, $H _2 S$ gas is passed through an acidified solution containing these ions in order to test $Cu^{2+}$ alone. Explain briefly.
(1998, 2M)
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Solution:
- $K _{\text {sp }}$ (solubility product) of $CuS$ is less than $K _{\text {sp }}$ of $ZnS$. On passing $H _2 S(g)$ in acidic solution, dissociation of $H _2 S$ is suppressed due to common ion effect and it provide small concentration of $S^{2-}$, sufficient for precipitation of $CuS$ but insufficient for precipitation of $ZnS$.
