Give the decreasing `pH` of aqueous solution of the following compounds :
a. `NaCl` , b. `MgCl_(2)` , c. `AlCl_(3)` , d. `PCl_(5)`

Options

A. `NaCl_(2) lt MgCl_(2) lt AlCl_(3)
B. `MgCl_(2) lt NaCl lt AlCl_(3)`
C. `AlCl_(3) lt MgCl_(2) lt NaCl`
D. `NaCl lt AlCl_(3) lt MgCl_(2)`

Explanation

To determine the decreasing pH of the aqueous solutions of the given compounds (NaCl, MgCl₂, AlCl₃, and PCl₅), we need to analyze the behavior of each compound in water, particularly focusing on the nature of their cations and the resulting acidity or basicity of the solutions.

### Step-by-Step Solution:

1. **Identify the Compounds and Their Ions**:
- NaCl dissociates into Na⁺ and Cl⁻.
- MgCl₂ dissociates into Mg²⁺ and Cl⁻.
- AlCl₃ dissociates into Al³⁺ and Cl⁻.
- PCl₅ dissociates into PCl₄⁺ and Cl⁻.

2. **Analyze the Cations**:
- Na⁺: This ion does not hydrolyze in water and does not affect the pH significantly. It is neutral.
- Mg²⁺: This ion can hydrolyze to form a weak acid (Mg(OH)₂), slightly lowering the pH.
- Al³⁺: This ion hydrolyzes more strongly than Mg²⁺, forming Al(OH)₃, which is a stronger acid than Mg(OH)₂, thus lowering the pH further.
- PCl₄⁺: This ion is a strong acid and will significantly lower the pH of the solution.

3. **Determine the pH Order**:
- Since Na⁺ is neutral, it will have the highest pH.
- Mg²⁺ will lower the pH slightly compared to NaCl.
- Al³⁺ will lower the pH more than Mg²⁺.
- PCl₄⁺ will result in the lowest pH due to its strong acidic nature.

4. **Final Order of Decreasing pH**:
- NaCl > MgCl₂ > AlCl₃ > PCl₅

### Conclusion:
The decreasing order of pH for the aqueous solutions of the given compounds is:
- **NaCl > MgCl₂ > AlCl₃ > PCl₅**