Which of the following mixtures will act as buffer?

Options

A. `H_(2)CO_(3)+NaOH(1.5:1 "molar ratio")`
B. `H_(2)CO_(3)+NaOH(1.5:2 "molar ratio")`
C. `NH_(4)OH+HCl(5:4 "molar ratio")`
D. `NH_(4)OH+HCl(4:5 "molar ratio")`

Answer

To determine which mixtures will act as buffers, we need to understand the conditions required for a solution to function as a buffer. A buffer solution typically consists 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 Each Mixture**:
- **Option A**: H2CO3 (carbonic acid) and NaOH (sodium hydroxide)
- **Option B**: H2CO3 and NaOH (in a different ratio)
- **Option C**: NH4OH (ammonium hydroxide) and NH4Cl (ammonium chloride)
- **Option D**: NH4OH and HCl (hydrochloric acid)

2. **Analyze Option A (H2CO3 and NaOH)**:
- H2CO3 is a weak acid, and when reacted with NaOH (a strong base), it produces NaHCO3 (sodium bicarbonate) and water.
- The resulting solution contains both a weak acid (H2CO3) and its conjugate base (NaHCO3).
- **Conclusion**: This mixture acts as a buffer.

3. **Analyze Option B (H2CO3 and NaOH in a different ratio)**:
- Similar to Option A, H2CO3 reacts with NaOH to form NaHCO3 and water.
- The presence of both H2CO3 (weak acid) and NaHCO3 (conjugate base) confirms that this mixture also acts as a buffer.
- **Conclusion**: This mixture acts as a buffer.

4. **Analyze Option C (NH4OH and NH4Cl)**:
- NH4OH is a weak base, and NH4Cl is the salt formed from a strong acid (HCl) and the weak base (NH4OH).
- The presence of NH4OH (weak base) and NH4Cl (conjugate acid) indicates that this mixture can also function as a buffer.
- **Conclusion**: This mixture acts as a buffer.

5. **Analyze Option D (NH4OH and HCl)**:
- In this case, NH4OH (weak base) reacts with HCl (strong acid) to form NH4Cl and water.
- However, since HCl is a strong acid, it will completely dissociate, and there will be no weak base left in the solution to act as a buffer.
- **Conclusion**: This mixture does not act as a buffer.

### Final Answer:
The mixtures that will act as buffers are **Option A, Option B, and Option C**.