Question

The metal ion (M$^{2+}$) in the following reaction is
\[ \text{M}^{2+} + \text{S}^{2-} \longrightarrow \text{Black precipitate} \xrightarrow{\text{hot conc. HNO}_3} \text{White precipitate} \]

• A. Mn$^{2+}$
• B. Fe$^{2+}$
• C. Cd$^{2+}$
• D. Cu$^{2+}$

Hint:

CdS is a black solid that turns to white sulfur upon oxidation with hot nitric acid.

Answer 1

The Correct Option is D

Step 1: Identify the black precipitate.
Cd$^{2+}$ + S$^{2-}$ → CdS (black precipitate).
Other metal sulfides like MnS and FeS are not pure black, and CuS is black but behaves differently upon oxidation.
Step 2: Reaction with hot concentrated HNO$_3$.
CdS + HNO$_3$ (hot) → CdSO$_4$ + S (white solid).
This is characteristic for CdS, producing white sulfur after oxidation.
Step 3: Conclusion.
Thus, the metal ion is Cd$^{2+}$.