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: \( \text{NiS} \).

Answer 2

Step 1: Analyze the first reaction.
The compound \( A \) is reacted with aqua regia. Aqua regia is a mixture of hydrochloric acid (HCl) and nitric acid (HNO₃), which is a powerful oxidizing agent. It is capable of dissolving metals like nickel, gold, and platinum. Therefore, it is likely that \( A \) contains nickel (Ni), as aqua regia typically dissolves metals like nickel to form their salts.

Step 2: Analyze the second reaction.
The compound \( B \) (formed from the reaction of \( A \) with aqua regia) is treated with potassium nitrate (KNO₃) and ammonium hydroxide (NH₄OH). This reaction leads to the formation of a yellow precipitate. A yellow precipitate is characteristic of nickel(II) hydroxide (\( \text{Ni(OH)}_2 \)) or nickel(II) sulfide (\( \text{NiS} \)) under certain conditions.

Step 3: Identify the compound \( A \).
Since the yellow precipitate formed is likely to be \( \text{NiS} \) (nickel sulfide), the compound \( A \) is most likely \( \text{NiS} \), as it can react with aqua regia to form a soluble nickel salt, which then precipitates as \( \text{NiS} \) under the given conditions.

Final Answer:
\[ \boxed{\text{NiS}}. \]