In qualitative analysis when H_2S is passed through an aqueous so…

MCQ

In qualitative analysis when $\ce{H_2S}$ is passed through an aqueous solution of salt acidified with dil$. \text{HCl},$ a black precipitate is obtained. On boiling the precipitate with dil$. \ce{HNO_3},$ it forms a solution of blue colour. Addition of excess of aqueous solution of ammonia to this solution gives $......$

A
Deep blue precipitate of $\mathrm{Cu}(\mathrm{OH})_2$.
✓
Deep blue solution of $\left[\mathrm{Cu}\left(\mathrm{NH}_3\right)_4\right]^{2+}$.
C
Deep blue solution of $\mathrm{Cu}\left(\mathrm{NO}_3\right)_2$.
D
Deep blue solution of $\mathrm{Cu}(\mathrm{OH})_2 \cdot \mathrm{Cu}\left(\mathrm{NO}_3\right)_2$.

✓

Answer

Correct option: B.

Deep blue solution of $\left[\mathrm{Cu}\left(\mathrm{NH}_3\right)_4\right]^{2+}$.

Black precipitate of copper sulphide is formed which gives blue colour of copper nitrate on boiling with dilute $\ce{HNO_3}.$ When aqueous solution of ammonia is added to it, deep blue colour of
$\text{Cu}^{2+}+\text{H}_2\text{S}\ \xrightarrow{\ \ \ \ \ \ \ \ \ }\ \text{CuS}+2\text{H}^+,\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{Black}$
$\text{CuS}+\text{dil. }\text{HNO}_3\ \xrightarrow{\ \ \ \ \ \ \ \ \ \ }\ \text{Cu}(\text{NO}_3)_2\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{Blue}$
$\text{Cu}(\text{NO}_3)_2+4\text{NH}_3\xrightarrow{\ \ \ \ \ \ \ \ \ \ }[\text{Cu}(\text{NH}_3)_4]^{2+}\\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \text{Deep blue}$