The pH of `0.1` M solution of the following increases in the order

Options

A. `NaCl lt NH_(4)Cl lt NaCN lt HCl`
B. `HCl lt NH_(4)Cl lt NaCl lt NaCN`
C. `NaCN lt NH_(4)Cl lt NaCl lt HCl`
D. `HCl lt NaCl lt NaCN lt NH_(4)Cl`

Answer

To determine the order of pH for the given solutions, we need to analyze the nature of each compound in the solution and how they affect the concentration of H⁺ ions in the solution.

### Step-by-Step Solution:

1. **Identify the Compounds**: The compounds given are NaCl, Ammonium Chloride (NH₄Cl), Sodium Cyanide (NaCN), and Hydrochloric Acid (HCl).

2. **Understand the Nature of Each Compound**:
- **HCl**: This is a strong acid that completely dissociates in water to produce H⁺ ions. Therefore, it will have the lowest pH.
- **NaCl**: This is a salt formed from a strong acid (HCl) and a strong base (NaOH). It does not affect the pH of the solution significantly and will have a neutral pH of around 7.
- **NH₄Cl**: This is a salt formed from a weak base (NH₃) and a strong acid (HCl). It will produce some H⁺ ions in solution, making it slightly acidic, so its pH will be less than 7.
- **NaCN**: This is a salt formed from a strong base (NaOH) and a weak acid (HCN). It will produce OH⁻ ions in solution, making it basic, so its pH will be greater than 7.

3. **Determine the pH Order**:
- **Lowest pH**: HCl (strong acid) → pH < 1
- **Next Lowest pH**: NH₄Cl (acidic salt) → pH < 7
- **Neutral pH**: NaCl (neutral salt) → pH ≈ 7
- **Highest pH**: NaCN (basic salt) → pH > 7

4. **Final Order of pH**:
- HCl < NH₄Cl < NaCl < NaCN

### Conclusion:
The pH of the solutions increases in the order:
**HCl < NH₄Cl < NaCl < NaCN**.