Which mixed sulphate is not an alum:

Options

A. `K_(2)SO_(4).Al_(2)(SO_(4))_(3).24H_(2)O`
B. `K_(2)SO_(4).Cr_(2)(SO_(4))_(3).24H_(2)O`
C. `Na_(2)SO_(4).Fe_(2)(SO_(4))_(3).24H_(2)O`
D. `CuSO_(4).Al_(2)(SO_(4))_(3).24H_(2)O`

Answer

To determine which mixed sulfate is not an alum, we need to understand the definition of an alum and analyze each option provided.

### Step-by-Step Solution:

1. **Understanding Alum**:
- An alum is a type of double sulfate that can be represented by the general formula:
\[ M_2SO_4 \cdot M'_{2}(SO_4)_3 \cdot 24H_2O \]
- Here, \( M \) is a monovalent metal (oxidation state +1), and \( M' \) is a trivalent metal (oxidation state +3).

2. **Analyzing Each Option**:
- **Option 1**: \( K_2SO_4 \cdot Al_2(SO_4)_3 \cdot 24H_2O \)
- Potassium (K) has an oxidation state of +1.
- Aluminium (Al) has an oxidation state of +3.
- This fits the alum criteria.

- **Option 2**: \( K_2SO_4 \cdot Cr_2(SO_4)_3 \cdot 24H_2O \)
- Potassium (K) has an oxidation state of +1.
- Chromium (Cr) has an oxidation state of +3.
- This also fits the alum criteria.

- **Option 3**: \( Na_2SO_4 \cdot Fe_2(SO_4)_3 \cdot 24H_2O \)
- Sodium (Na) has an oxidation state of +1.
- Iron (Fe) is in the +3 oxidation state (ferric).
- This fits the alum criteria as well.

- **Option 4**: \( CuSO_4 \cdot Al_2(SO_4)_3 \cdot 24H_2O \)
- Copper (Cu) has an oxidation state of +2.
- Aluminium (Al) has an oxidation state of +3.
- Here, copper does not have the required +1 oxidation state, thus it does not fit the alum criteria.

3. **Conclusion**:
- Based on the analysis, the mixed sulfate that is not an alum is **Option 4: \( CuSO_4 \cdot Al_2(SO_4)_3 \cdot 24H_2O \)**.

### Final Answer:
The mixed sulfate that is not an alum is **Option 4**.