Which one of these is not an acid salt ?

Options

A. `NaH_2PO_2`
B. `NaH_2PO_3`
C. `Na_2H_2S_2O_7`
D. `NaH_2PO_4`

Answer

To determine which of the given options is not an acid salt, we need to understand the concept of acid salts and how they differ from other types of salts.

### Step-by-Step Solution:

1. **Definition of Acid Salt**: An acid salt is formed when a strong acid reacts with a weak base or when a weak acid reacts with a strong base. Acid salts contain one or more replaceable hydrogen ions (H+) in their structure.

2. **Identify the Options**: The options provided are:
- A) NaH2PO2
- B) NaH2PO3
- C) Na2H2SO2O7
- D) NaH2PO4

3. **Analyze Each Option**:
- **Option A: NaH2PO2**: This is a salt formed from the weak acid H2PO2 and a strong base (NaOH). It has one replaceable H+ ion, indicating it is an acid salt.
- **Option B: NaH2PO3**: This is formed from the weak acid H3PO3 and a strong base. It has one replaceable H+ ion, indicating it is also an acid salt.
- **Option C: Na2H2SO2O7**: This is a salt derived from a weak acid (H2SO2O7) and a strong base. It has two replaceable H+ ions, indicating it is an acid salt.
- **Option D: NaH2PO4**: This is formed from the weak acid H3PO4 and a strong base. It has one replaceable H+ ion, indicating it is an acid salt.

4. **Determine the Non-Acid Salt**: Among the options, NaH2PO2 is actually a basic salt because it is derived from a weak acid and a strong base. The aqueous solution of NaH2PO2 is alkaline in nature.

5. **Conclusion**: Therefore, the option that is not an acid salt is **A) NaH2PO2**.

### Final Answer:
The correct option that is not an acid salt is **A) NaH2PO2**.