An acidic buffer is obtained on mixing :

Options

A. 100 mL of 0.1 M HCl and 200 mL of 0.1 M `CH_(3)COONa`
B. 100 mL of 0.1 M HCl and 200 mL of 0.1 M NaCl
C. 100 mL of 0.1 M `CH_(3) COOH` and 100 mL of 0.1 M NaOH
D. 100 mL of 0.1 M `CH_(3)COOH` and 200 mL of 0.1 M NaOH

Explanation

To solve the question regarding the composition of an acidic buffer, we need to understand the basic concept of what an acidic buffer is and how it is formed. An acidic buffer is typically made by mixing a weak acid with its salt (which is the conjugate base of that weak acid). The pH of an acidic buffer is less than 7.

### Step-by-Step Solution:

1. **Understanding Acidic Buffers**:
- An acidic buffer is formed by mixing a weak acid with its salt (the conjugate base). The pH of the solution remains relatively constant when small amounts of acid or base are added.

2. **Identifying the Components**:
- We need to analyze the given options to determine which combination of acid and salt will yield an acidic buffer.
- The options typically include a strong acid, weak acid, strong base, and their respective salts.

3. **Analyzing the First Option**:
- The first option is HCl (a strong acid) and CH3COONa (the salt of acetic acid, which is a weak acid).
- HCl will dissociate completely in solution, providing H⁺ ions, while CH3COONa will dissociate to give CH3COO⁻ ions (the conjugate base of acetic acid).

4. **Calculating Milliequivalents**:
- For HCl:
- n factor = 1 (since it gives 1 H⁺)
- Molarity = 0.1 M
- Volume = 100 mL
- Milliequivalents = n factor × molarity × volume = 1 × 0.1 × 100 = 10 mEq.
- For CH3COONa:
- n factor = 1 (it provides 1 CH3COO⁻)
- Molarity = 0.1 M
- Volume = 200 mL
- Milliequivalents = 1 × 0.1 × 200 = 20 mEq.

5. **Resulting Composition**:
- Mixing 10 mEq of HCl with 20 mEq of CH3COONa will yield a solution containing excess CH3COO⁻ ions, which can react with any added H⁺ ions, thus maintaining the pH.

6. **Conclusion**:
- Since we have a weak acid (acetic acid from CH3COONa) and its salt, this combination will act as an acidic buffer. Therefore, the first option is correct.

### Final Answer:
The correct option for obtaining an acidic buffer is the first option: HCl and CH3COONa.