The salt having the least solubility in water is :

Options

A. `BaCl_(2)`
B. `Ba(NO_(3))_(2)`
C. `MgSO_(4)`
D. `BaSO_(4)`

Answer

To determine which salt has the least solubility in water among the given options (BaCl2, Ba(NO3)2, MgSO4, and BaSO4), we can analyze the solubility of each salt based on their lattice enthalpy and hydration enthalpy.

### Step-by-Step Solution:

1. **Understanding Solubility**:
- Solubility in water is influenced by two main factors: lattice enthalpy and hydration enthalpy.
- Lattice enthalpy is the energy required to separate one mole of a solid ionic compound into its gaseous ions. Higher lattice enthalpy generally means lower solubility.
- Hydration enthalpy is the energy released when gaseous ions are surrounded by water molecules. Higher hydration enthalpy generally means higher solubility.

2. **Comparing the Salts**:
- **BaCl2 (Barium Chloride)**: This salt is quite soluble in water due to the relatively low lattice enthalpy and high hydration enthalpy of the barium and chloride ions.
- **Ba(NO3)2 (Barium Nitrate)**: Similar to BaCl2, it is also soluble in water. The nitrate ion has a high hydration enthalpy.
- **MgSO4 (Magnesium Sulfate)**: This salt is soluble in water, but its solubility is lower than that of BaCl2 and Ba(NO3)2 due to the higher lattice enthalpy of MgSO4 compared to the barium salts.
- **BaSO4 (Barium Sulfate)**: This salt has a very low solubility in water. The lattice enthalpy of BaSO4 is significantly high, and its hydration enthalpy is not sufficient to overcome the lattice energy.

3. **Conclusion**:
- Among the four salts, BaSO4 has the highest lattice enthalpy relative to its hydration enthalpy, making it the least soluble in water.
- Therefore, the salt with the least solubility in water is **BaSO4** (option D).

### Final Answer:
The salt having the least solubility in water is **BaSO4**.