Which of the following pairs of lons cannot be separated by `H_2S` in ammoniacal medium?

Options

A. `Zn^(2+)`, `Ni^(2+)`
B. `Zn^(2+)Mg^(2+)`
C. `Mn^(2+), Ca^(2+)`
D. `Co^(2+),Ca^(2+)`,

Answer

To solve the question of which pairs of ions cannot be separated by H₂S in an ammoniacal medium, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Medium**: Recognize that an ammoniacal medium is basic in nature. This is important because the behavior of certain metal ions in a basic medium will determine whether they will precipitate as sulfides when treated with H₂S.

2. **Identify the Ions**: List the ions provided in the question. For the sake of this solution, let's assume the ions are Zinc (Zn²⁺), Nickel (Ni²⁺), Magnesium (Mg²⁺), and Calcium (Ca²⁺).

3. **Know the Precipitation Behavior**: In a basic medium, certain metal ions form precipitates with H₂S:
- **Zinc (Zn²⁺)** forms a white precipitate of Zinc Sulfide (ZnS).
- **Nickel (Ni²⁺)** forms a black precipitate of Nickel Sulfide (NiS).
- **Magnesium (Mg²⁺)** does not form a precipitate with H₂S in a basic medium.
- **Calcium (Ca²⁺)** also does not form a precipitate with H₂S in a basic medium.

4. **Analyze the Pairs**: Since we are looking for pairs of ions that cannot be separated by H₂S:
- Zinc and Nickel can be separated because they form different colored precipitates (white for ZnS and black for NiS).
- Magnesium and Calcium do not form precipitates with H₂S, hence they cannot be separated by this method.

5. **Conclusion**: The pair of ions that cannot be separated by H₂S in an ammoniacal medium is Magnesium (Mg²⁺) and Calcium (Ca²⁺).

### Final Answer:
The pair of ions that cannot be separated by H₂S in an ammoniacal medium is **Magnesium (Mg²⁺) and Calcium (Ca²⁺)**.

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