Solubility of the alkaline earth's metal sulphates in water decreases in the sequence

Options

A. `Ba gt Mg gt Sr gt Ca`
B. `Mg gt Ca gt Sr gt Ba`
C. `Ca gt Sr gt Ba gt Mg`
D. `Sr gt Ca gt Mg gt Ba`

Answer

To determine the solubility of alkaline earth metal sulfates in water, we need to analyze the factors influencing their solubility, particularly focusing on the hydration enthalpy of the ions involved.

### Step-by-Step Solution:

1. **Identify the Alkaline Earth Metals and Their Sulfates**:
- The alkaline earth metals include magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba).
- The sulfates of these metals are MgSO₄, CaSO₄, SrSO₄, and BaSO₄.

2. **Understand the Role of Hydration Energy**:
- Hydration energy is the energy released when ions are surrounded by water molecules. It is a crucial factor in determining the solubility of ionic compounds in water.
- Higher hydration energy typically leads to greater solubility.

3. **Analyze the Trend in Cation Size**:
- As we move down the group from magnesium to barium, the size of the cations increases (Mg²⁺ < Ca²⁺ < Sr²⁺ < Ba²⁺).
- Larger cations have lower charge density, which results in weaker interactions with water molecules.

4. **Evaluate the Effect on Hydration Energy**:
- With increasing size of the cations, the hydration energy decreases. This means that the ability of the sulfate ions to be solvated (surrounded by water) diminishes.
- Consequently, the solubility of the sulfates decreases as we go from magnesium to barium.

5. **Establish the Sequence of Solubility**:
- Based on the hydration energy trend, we can conclude that:
- MgSO₄ (most soluble) > CaSO₄ > SrSO₄ > BaSO₄ (least soluble).
- Therefore, the correct sequence of solubility of alkaline earth metal sulfates in water is:
- **MgSO₄ > CaSO₄ > SrSO₄ > BaSO₄**.

### Final Answer:
The solubility of alkaline earth metal sulfates in water decreases in the sequence: **MgSO₄ > CaSO₄ > SrSO₄ > BaSO₄**.