Which of the following is least soluble in water?

Options

A. `MgSO_4`
B. `CaSO_4`
C. BeSO_4
D. `BaSO_4`

Answer

To determine which of the following compounds is least soluble in water, we need to consider the concepts of lattice energy and hydration energy.

### Step-by-Step Solution:

1. **Understanding Lattice Energy and Hydration Energy**:
- Lattice energy is the energy required to separate one mole of a solid ionic compound into its gaseous ions. Higher lattice energy means the compound is less soluble.
- Hydration energy is the energy released when ions are surrounded by water molecules. Higher hydration energy means the compound is more soluble.

2. **Analyzing the Compounds**:
- The question likely involves compounds from the alkaline earth metals (Group 2) such as barium sulfate (BaSO₄), magnesium sulfate (MgSO₄), calcium sulfate (CaSO₄), etc.
- As we move down the group from magnesium to barium, the size of the ions increases.

3. **Trends in Solubility**:
- In Group 2, as the size of the cation increases (from Mg²⁺ to Ba²⁺), the lattice energy changes very little, but the hydration energy decreases significantly.
- Smaller ions (like Mg²⁺) have a higher charge density, leading to stronger interactions with water molecules, thus higher hydration energy.

4. **Comparing the Solubility**:
- Magnesium sulfate (MgSO₄) is more soluble due to its higher hydration energy.
- Calcium sulfate (CaSO₄) has lower solubility than magnesium sulfate but is more soluble than barium sulfate.
- Barium sulfate (BaSO₄) has the largest cation and the lowest hydration energy, making it the least soluble among these compounds.

5. **Conclusion**:
- Based on the analysis, barium sulfate (BaSO₄) is the least soluble in water among the given options.

### Final Answer:
**Barium sulfate (BaSO₄) is the least soluble in water.**