Aqueous solution of which of the following cation gives precipitate with potash alum?

Options

A. `Cu^(2+)(aq.)`
B. `Zn^(2+)(aq.)`
C. `Ba^(2+)(aq.)`
D. `Ni^(2+)(aq.)`

Answer

To determine which cation gives a precipitate with potash alum, we need to analyze the components of potash alum and the solubility of the potential cations with sulfate ions.

### Step-by-Step Solution:

1. **Identify the Composition of Potash Alum**:
- Potash alum has the formula: K₂SO₄ · Al₂(SO₄)₃ · 24H₂O.
- The key anion in potash alum is the sulfate ion (SO₄²⁻).

2. **Understand Precipitate Formation**:
- A precipitate forms when a soluble cation reacts with an anion to produce an insoluble compound.
- In this case, we are looking for a cation that forms an insoluble sulfate.

3. **Evaluate the Given Cations**:
- We need to check the solubility of the sulfate salts of the given cations:
- **Copper (Cu²⁺)**: Copper sulfate (CuSO₄) is soluble.
- **Zinc (Zn²⁺)**: Zinc sulfate (ZnSO₄) is also soluble.
- **Barium (Ba²⁺)**: Barium sulfate (BaSO₄) is insoluble.
- **Nickel (Ni²⁺)**: Nickel sulfate (NiSO₄) is soluble.

4. **Identify the Cation that Forms an Insoluble Sulfate**:
- From the analysis, barium (Ba²⁺) is the only cation that forms an insoluble sulfate (BaSO₄).
- Therefore, barium will give a precipitate when reacted with potash alum.

5. **Conclusion**:
- The cation that gives a precipitate with potash alum is **Barium (Ba²⁺)**.

### Final Answer:
The aqueous solution of **Barium (Ba²⁺)** gives a precipitate with potash alum.

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