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 can follow these steps:

### Step 1: Understand the Composition of Potash Alum
Potash alum has the chemical formula K₂SO₄·Al₂(SO₄)₃·24H₂O. The key anion here is the sulfate ion (SO₄²⁻).

### Step 2: Identify the Cations Provided
We need to evaluate the cations given in the options. Let's assume the options are:
1. Copper (Cu²⁺)
2. Zinc (Zn²⁺)
3. Barium (Ba²⁺)
4. Nickel (Ni²⁺)

### Step 3: Check Solubility of the Corresponding Sulfates
We will check the solubility of the sulfates formed with each cation:

1. **Copper (Cu²⁺)**: Copper sulfate (CuSO₄) is soluble in water.
2. **Zinc (Zn²⁺)**: Zinc sulfate (ZnSO₄) is also soluble in water.
3. **Barium (Ba²⁺)**: Barium sulfate (BaSO₄) is insoluble in water.
4. **Nickel (Ni²⁺)**: Nickel sulfate (NiSO₄) is soluble in water.

### Step 4: Identify the Cation that Forms an Insoluble Sulfate
From the analysis above, we see that:
- Barium sulfate (BaSO₄) is the only sulfate that is insoluble in water. Therefore, barium (Ba²⁺) will give a precipitate when mixed with potash alum.

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

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