Which one of the following mixtures is suitable for making a buffer solution will an optimum pH of 9.2 - 9.3 ?

Options

A. `CH_3CO_2Na//CH_3CO_2H (K_a = 1.8 xx10^(-5))`
B. `NH_3//NH_4Cl (K_a(NH_4^(+))=5.6 xx10^(-10)) `
C. `NaNO_2//HOCl(K_a = 4.5 xx10^(-8))`
D. `NaNO_2//HNO_2(K_a = 4.5xx10^(-4))`

Answer

To determine which mixture is suitable for making a buffer solution with an optimum pH of 9.2 - 9.3, we need to understand the components of a basic buffer solution. A basic buffer is typically composed of a weak base and its salt with a strong acid.

### Step-by-Step Solution:

1. **Identify the pH Range**:
- The given pH range is 9.2 to 9.3, which indicates that we are looking for a basic buffer solution.

2. **Understand Basic Buffers**:
- A basic buffer is formed from a weak base and its corresponding salt with a strong acid. This means we need to look for a weak base and a salt derived from a strong acid.

3. **Evaluate the Options**:
- **Option A**: CH3CO2Na (Sodium acetate) and CH3COOH (Acetic acid)
- This combination is an acidic buffer because it consists of a weak acid (acetic acid) and its salt (sodium acetate). Therefore, this option is not suitable.

- **Option B**: NH3 (Ammonia) and NH4Cl (Ammonium chloride)
- NH3 is a weak base, and NH4Cl is the salt of the weak base (NH3) and a strong acid (HCl). This combination is suitable for a basic buffer. Thus, this option is correct.

- **Option C**: NaNO2 (Sodium nitrite) and HOCl (Hypochlorous acid)
- This mixture consists of a salt of a weak acid and a strong base, making it an acidic buffer. Therefore, this option is not suitable.

- **Option D**: NaNO2 (Sodium nitrite) and HNO2 (Nitrous acid)
- This combination also represents an acidic buffer because HNO2 is a weak acid, and NaNO2 is its salt. Hence, this option is not suitable.

4. **Conclusion**:
- After evaluating all options, the only suitable mixture for making a buffer solution with an optimum pH of 9.2 - 9.3 is **Option B: NH3 and NH4Cl**.

### Final Answer:
**Option B: NH3 and NH4Cl**