Which of the following is not a buffer?

Options

A. `NH_(4)OH + C_(6)H_(5)COOH`
B. `HCOOH + HCOONa`
C. `H_(2)CO_(3) + HCOOK`
D. `NH_(4)OH +(NH_(4))_(2)SO_(4)`

Explanation

To determine which of the following options is not a buffer, we first need to understand what a buffer is. A buffer solution is a system that can resist changes in pH upon the addition of small amounts of an acid or a base. There are two types of buffers:

1. **Acidic Buffer**: This consists of a weak acid and its salt with a strong base.
2. **Basic Buffer**: This consists of a weak base and its salt with a strong acid.

Now, let's analyze each option provided in the question:

### Step-by-Step Solution:

1. **Option 1: NH4OH + C6H5COOH**
- NH4OH (ammonium hydroxide) is a weak base.
- C6H5COOH (benzoic acid) is a weak acid.
- However, this combination does not form a buffer because it lacks the corresponding salt (the salt of the weak acid or weak base). Thus, this option does not represent a buffer.

2. **Option 2: HCOOH + HCOONA**
- HCOOH (formic acid) is a weak acid.
- HCOONA (sodium formate) is the salt of formic acid and a strong base (NaOH).
- This combination can form a buffer solution since it consists of a weak acid and its salt.

3. **Option 3: HCO3 + HCOOK**
- HCO3 (bicarbonate) is a weak acid.
- HCOOK (potassium formate) is the salt of the weak acid and a strong base (KOH).
- This combination can also form a buffer solution.

4. **Option 4: NH4OH + (NH4)2SO4**
- NH4OH (ammonium hydroxide) is a weak base.
- (NH4)2SO4 (ammonium sulfate) is the salt formed from NH4OH and a strong acid (H2SO4).
- This combination can also form a buffer solution.

### Conclusion:
From the analysis, we find that **Option 1 (NH4OH + C6H5COOH)** is not a buffer solution.

### Final Answer:
**The option that is not a buffer is: NH4OH + C6H5COOH.**

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