When an ionic compound is dissolved in water (polar solvent), it breaks up into its constituent ions. The given ionic compound will be dissolved in water if its hydration energy is more than lattice energy . IF hydration energy is less than lattice energy then ionic compound is usually either sparingly soluble or insoluble in water.
Most hydrated cation is :

Options

A. `Ce_((aq.))^(4+)`
B. `La_((aq.))^(3+)`
C. `Ba_((aq.))^(2+)`
D. `CS_((aq))^(+)`

Answer

### Step-by-Step Solution:

1. **Understanding Hydration Energy and Lattice Energy**:
- When an ionic compound is dissolved in water, it dissociates into its constituent ions. The process involves two key energies: hydration energy (the energy released when water molecules surround and stabilize the ions) and lattice energy (the energy required to separate the ions in the solid lattice).
- For an ionic compound to dissolve, the hydration energy must be greater than the lattice energy.

2. **Identifying the Most Hydrated Cation**:
- The degree of hydration of a cation depends on its charge and size. Generally, cations with higher positive charges and smaller radii have a greater ability to attract water molecules, leading to higher hydration energy.
- The cation that is most hydrated will be the one that can interact most effectively with water molecules.

3. **Evaluating Cations**:
- Among common cations, we can evaluate their hydration based on their charge and size. For example, cations like \( \text{Li}^+ \), \( \text{Na}^+ \), \( \text{K}^+ \), \( \text{Ca}^{2+} \), and \( \text{Al}^{3+} \) can be considered.
- However, \( \text{Ce}^{4+} \) (Cerium ion) is notable because it has a higher charge (+4) compared to others, which means it can attract more water molecules.

4. **Conclusion**:
- Given that \( \text{Ce}^{4+} \) has a high charge and is relatively small in size, it will have a high hydration energy due to the strong interaction with water molecules. Thus, it is considered the most hydrated cation.

### Final Answer:
The most hydrated cation is \( \text{Ce}^{4+} \).

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