Which may be added to one litre of water to act as a buffer

Options

A. One mole of `HC_(2)H_(3)O_(2)` and 0.5 mole of NaOH
B. One mole of `NH_(4)Cl` and one mole of HCl
C. One mole of `NH_(4)OH` and one mole of NaOH
D. One mole of `HC_(2)H_(3)O_(2)` and one mole of HCl

Explanation

To determine which substance can be added to one liter of water to act as a buffer, we need to understand the concept of a buffer solution. A buffer solution is typically made up of a weak acid and its conjugate base or a weak base and its conjugate acid.

### Step-by-Step Solution:

1. **Identify the Components of a Buffer:**
- A buffer must consist of a weak acid and its conjugate base, or a weak base and its conjugate acid.

2. **Examine the Options:**
- We need to analyze the given options to see which one contains a weak acid and its conjugate base or a weak base and its conjugate acid.

3. **Option Analysis:**
- **Option 1:** Acetic acid (CH₃COOH) and sodium hydroxide (NaOH).
- Acetic acid is a weak acid, and when it reacts with NaOH, it forms sodium acetate (the conjugate base). This combination can act as a buffer.
- **Option 2:** Ammonium chloride (NH₄Cl).
- This is a salt of a strong acid (HCl) and a weak base (NH₃). It does not provide a weak acid and its conjugate base.
- **Option 3:** Hydrochloric acid (HCl).
- HCl is a strong acid and cannot form a buffer with any conjugate base.
- **Option 4:** Ammonium hydroxide (NH₄OH) and sodium hydroxide (NaOH).
- This combination does not provide a weak acid and its conjugate base.

4. **Conclusion:**
- The only option that can form a buffer solution is **Option 1**, which contains acetic acid and sodium hydroxide. This mixture will yield a weak acid (acetic acid) and its conjugate base (sodium acetate), thus acting as a buffer.

### Final Answer:
**Acetic acid (CH₃COOH) and sodium hydroxide (NaOH) can be added to one liter of water to act as a buffer.**