How many of the following combination act as buffer
(1) `HCl+NaOH`
(2) `CH_3COONa+CH_3COOH`
(3) `H_2SO_4+Na_2SO_4`
(4) `H_2CO_3+NaOH`
(5) `Na_2B_4O_7+H_3BO_3`
(6) `NH_4OH +NH_4Cl`

Options

A. 3
B. 4
C. 2
D. 6

Answer

To determine how many of the given combinations act as buffers, we need to understand the definition of a buffer solution. A buffer is typically composed of a weak acid and its conjugate base or a weak base and its conjugate acid.

Let's analyze each combination step by step:

1. **HCl + NaOH**:
- HCl is a strong acid and NaOH is a strong base.
- Since both components are strong, they do not form a buffer solution.
- **Conclusion**: This combination does not act as a buffer.

2. **CH₃COONa + CH₃COOH**:
- CH₃COOH (acetic acid) is a weak acid, and CH₃COONa (sodium acetate) is its conjugate base.
- This combination fits the definition of a buffer.
- **Conclusion**: This combination acts as a buffer.

3. **H₂SO₄ + Na₂SO₄**:
- H₂SO₄ is a strong acid, and Na₂SO₄ is a salt derived from a strong acid.
- Since H₂SO₄ is a strong acid, this combination does not form a buffer.
- **Conclusion**: This combination does not act as a buffer.

4. **H₂CO₃ + NaOH**:
- H₂CO₃ (carbonic acid) is a weak acid, and NaOH is a strong base.
- When NaOH reacts with H₂CO₃, it forms NaHCO₃ (sodium bicarbonate), which is the conjugate base of H₂CO₃.
- This combination can act as a buffer.
- **Conclusion**: This combination acts as a buffer.

5. **Na₂B₄O₇ + H₃BO₃**:
- H₃BO₃ (boric acid) is a weak acid, and Na₂B₄O₇ (sodium tetraborate) can be considered its conjugate base.
- This combination fits the definition of a buffer.
- **Conclusion**: This combination acts as a buffer.

6. **NH₄OH + NH₄Cl**:
- NH₄OH (ammonium hydroxide) is a weak base, and NH₄Cl (ammonium chloride) is its conjugate acid.
- This combination also fits the definition of a buffer.
- **Conclusion**: This combination acts as a buffer.

Now, let's summarize the results:
- Combinations that act as buffers:
- (2) CH₃COONa + CH₃COOH
- (4) H₂CO₃ + NaOH
- (5) Na₂B₄O₇ + H₃BO₃
- (6) NH₄OH + NH₄Cl

Total combinations that act as buffers: **4**

### Final Answer:
**4 combinations act as buffers.**