Which of the following pairs of cations cannot be separated by adding `NH_(4)Cl` and `NH_(4)OH` to the mixture and then passing `H_(2)S` through it?

Options

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

Answer

To determine which pairs of cations cannot be separated by adding `NH₄Cl` and `NH₄OH` to the mixture and then passing `H₂S` through it, we need to analyze the behavior of the given cations when treated with these reagents.

### Step-by-Step Solution:

1. **Identify the Cations and Their Groups**:
- The cations provided in the question are:
- Pair 1: Calcium (Ca²⁺) and Nickel (Ni²⁺)
- Pair 2: Magnesium (Mg²⁺) and Manganese (Mn²⁺)
- Pair 3: Nickel (Ni²⁺) and Manganese (Mn²⁺)
- Pair 4: Cobalt (Co²⁺) and Zinc (Zn²⁺)

2. **Understand the Grouping of Cations**:
- In qualitative analysis, cations are grouped based on their solubility in certain reagents. The reagents `NH₄Cl` and `NH₄OH` are used to precipitate certain cations when `H₂S` is passed through the solution.
- The fourth group cations that are precipitated as sulfides include Ni²⁺, Mn²⁺, Co²⁺, and Zn²⁺.

3. **Analyze Each Pair**:
- **Pair 1: Ca²⁺ and Ni²⁺**:
- Ca²⁺ is a fifth group cation and will not precipitate with `H₂S`. Ni²⁺ is a fourth group cation and will precipitate as NiS.
- **Conclusion**: They can be separated.

- **Pair 2: Mg²⁺ and Mn²⁺**:
- Mg²⁺ is a sixth group cation and will not precipitate with `H₂S`. Mn²⁺ is a fourth group cation and will precipitate as MnS.
- **Conclusion**: They can be separated.

- **Pair 3: Ni²⁺ and Mn²⁺**:
- Both Ni²⁺ and Mn²⁺ are fourth group cations and will precipitate as NiS and MnS, respectively.
- **Conclusion**: They cannot be separated.

- **Pair 4: Co²⁺ and Zn²⁺**:
- Both Co²⁺ and Zn²⁺ are also fourth group cations and will precipitate as CoS and ZnS, respectively.
- **Conclusion**: They cannot be separated.

4. **Final Answer**:
- The pairs of cations that cannot be separated by adding `NH₄Cl` and `NH₄OH` and then passing `H₂S` through the mixture are:
- **Nickel (Ni²⁺) and Manganese (Mn²⁺)**
- **Cobalt (Co²⁺) and Zinc (Zn²⁺)**