A black precipitate is obtained when H2S is passed through a dilute acidic (HCI) aqueous solution containing

Options

A. `Cu^(2+)` and`Pb^(2+)`
B. `Zn^(2+)`and `Mg^(2+)`
C. `Mg^(2+)` and `Fe^(3+)`
D. `Mg^(2+)`and `Ni^(2+)`

Answer

To determine which ions produce a black precipitate when H2S is passed through a dilute acidic (HCl) aqueous solution, we can analyze the behavior of each ion with respect to H2S.

### Step-by-Step Solution:

1. **Identify the Ions**: The ions given in the question are Cu²⁺, Pb²⁺, Zn²⁺, Mg²⁺, Fe³⁺, and Ni²⁺.

2. **Consider the Reaction with H2S**: When H2S is introduced into a solution containing metal ions, it can form metal sulfides. The solubility of these sulfides varies depending on the metal ion.

3. **Analyze Each Ion**:
- **Zn²⁺**: Zinc sulfide (ZnS) is soluble in acidic solutions. Therefore, it does not produce a precipitate. **(Cancel this option)**
- **Fe³⁺**: Ferric ions (Fe³⁺) react with H2S to form ferrous sulfide (FeS), which is typically green, but the reaction can produce sulfur as a byproduct. No black precipitate is formed here. **(Cancel this option)**
- **Mg²⁺**: Magnesium sulfide (MgS) is also soluble in acidic solutions, so it does not produce a precipitate. **(Cancel this option)**
- **Cu²⁺**: Copper(II) sulfide (CuS) is insoluble and forms a black precipitate when H2S is added. **(Keep this option)**
- **Pb²⁺**: Lead(II) sulfide (PbS) is also insoluble and forms a black precipitate when H2S is added. **(Keep this option)**
- **Ni²⁺**: Nickel sulfide (NiS) can form a black precipitate, but it is not as commonly noted as CuS and PbS in this context. However, it is not the primary focus here.

4. **Conclusion**: The ions that produce a black precipitate when H2S is passed through a dilute acidic solution are Cu²⁺ and Pb²⁺. Therefore, the correct answer is both Cu²⁺ and Pb²⁺.

### Final Answer:
The black precipitate is formed by Cu²⁺ (copper(II) ions) and Pb²⁺ (lead(II) ions) when H2S is passed through a dilute acidic (HCl) aqueous solution.

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