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 what a buffer is. A buffer solution is one that resists changes in pH upon the addition of small amounts of acid or base. Typically, buffers consist of either a weak acid and its salt or a weak base and its salt.

Let's analyze each combination step by step:

1. **HCl + NaOH**:
- HCl is a strong acid and NaOH is a strong base. When mixed, they will completely neutralize each other to form water and NaCl. This combination does not act as a buffer.
- **Conclusion**: Not a buffer.

2. **CH₃COONa + CH₃COOH**:
- CH₃COOH (acetic acid) is a weak acid, and CH₃COONa (sodium acetate) is its salt. This combination can resist changes in pH when small amounts of acid or base are added.
- **Conclusion**: Acts as a buffer.

3. **H₂SO₄ + Na₂SO₄**:
- H₂SO₄ (sulfuric acid) is a strong acid, and Na₂SO₄ (sodium sulfate) is its salt. This combination does not provide the necessary weak acid and its salt to act as a buffer.
- **Conclusion**: Not a buffer.

4. **H₂CO₃ + NaOH**:
- H₂CO₃ (carbonic acid) is a weak acid, and if NaOH is added in limited amounts, it will partially neutralize to form NaHCO₃ (sodium bicarbonate), which is a salt of the weak acid. This combination can act as a buffer.
- **Conclusion**: Acts as a buffer.

5. **Na₂B₄O₇ + H₃BO₃**:
- H₃BO₃ (boric acid) is a weak acid, and Na₂B₄O₇ (sodium tetraborate) is its salt. This combination can also resist changes in pH, thus acting as a buffer.
- **Conclusion**: Acts as a buffer.

6. **NH₄OH + NH₄Cl**:
- NH₄OH (ammonium hydroxide) is a weak base, and NH₄Cl (ammonium chloride) is its salt. This combination can resist changes in pH and acts as a buffer.
- **Conclusion**: Acts as a buffer.

Now, let's summarize how many combinations act as buffers:

- **Buffer Combinations**:
- CH₃COONa + CH₃COOH (1)
- H₂CO₃ + NaOH (2)
- Na₂B₄O₇ + H₃BO₃ (3)
- NH₄OH + NH₄Cl (4)

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

### Summary of Buffers:
1. HCl + NaOH: Not a buffer
2. CH₃COONa + CH₃COOH: Buffer
3. H₂SO₄ + Na₂SO₄: Not a buffer
4. H₂CO₃ + NaOH: Buffer
5. Na₂B₄O₇ + H₃BO₃: Buffer
6. NH₄OH + NH₄Cl: Buffer

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