Question

Find the compound \( A \) from the following reaction sequences: \[ A \xrightarrow{\text{aqua regia}} B \xrightarrow{(1) \, \text{KNO}_3, \, \text{NH}_4\text{OH}} \text{yellow ppt}. \]

• A. \( \text{CoS} \)
• B. \( \text{NiS} \)
• C. \( \text{ZnS} \)
• D. \( \text{MnS} \)

Hint:

When performing precipitation reactions, always pay attention to the color of the precipitate as it often indicates the identity of the compound formed.

Answer 1

The Correct Option is B

- When a metal reacts with aqua regia (a mixture of hydrochloric acid and nitric acid), it forms metal ions.
- The subsequent treatment with \( \text{KNO}_3 \) and \( \text{NH}_4\text{OH} \) forms a precipitate.
- The yellow precipitate suggests the formation of \( \text{NiS} \) (nickel sulfide), which forms when nickel ions react with sulfide ions.

Thus, the compound \( A \) must be \( \text{Ni} \), which reacts with aqua regia to form \( \text{Ni}^{2+} \), and then precipitates as \( \text{NiS} \).

Final Answer: \( \text{NiS} \).