Alums having the general formula `M_(2)SO_(4).M_(2)^(1)(SO_(4))_(3).24H_(2)O` where M is monovalent basic radical while M is trivalent basic radical. Alums are generally obtained when hot solutions of equimolar quantities of their constituent sulphates are mixed and the resulting solution is subjected to crystallization. Alums are fairly soluble in hot water but less solubel in cold water
Which of the following cation can not form alum

Options

A. `Na^(+)`
B. `NH_(4)^(+)`
C. `Li^(+)`
D. `Cs^(+)`

Answer

To determine which cation cannot form an alum, we first need to understand the structure of alums and the requirements for their formation.

### Step-by-Step Solution:

1. **Understanding the General Formula of Alums**:
Alums have the general formula \( M_2SO_4 \cdot M'_{2}(SO_4)_{3} \cdot 24H_2O \), where:
- \( M \) is a monovalent cation (like \( K^+, Na^+, Li^+ \)).
- \( M' \) is a trivalent cation (like \( Al^{3+}, Fe^{3+} \)).

2. **Identifying Possible Cations**:
We need to consider common cations that are either monovalent or trivalent. Some common examples are:
- Monovalent: \( K^+, Na^+, Li^+, NH_4^+ \)
- Trivalent: \( Al^{3+}, Fe^{3+}, Cr^{3+} \)

3. **Analyzing the Cations for Alum Formation**:
- **Monovalent Cations**: \( K^+, Na^+, Li^+, NH_4^+ \) can all form alums with a trivalent cation.
- **Trivalent Cations**: \( Al^{3+}, Fe^{3+}, Cr^{3+} \) can also form alums with a monovalent cation.

4. **Identifying the Cation that Cannot Form Alum**:
We need to find a cation that does not fit the criteria for forming alums. An example of a cation that cannot form alum is:
- **Bivalent Cations**: \( Ca^{2+}, Mg^{2+} \) do not fit into the alum structure since they are not monovalent or trivalent.

5. **Conclusion**:
Based on the analysis, the cation that cannot form an alum is any bivalent cation, such as \( Ca^{2+} \) or \( Mg^{2+} \).

### Final Answer:
**Bivalent cations like \( Ca^{2+} \) cannot form alums.**