`H_2 SO_4` is added while preparing a standard solution of Mohr's salt to prevent ___.

Answer

To answer the question regarding why `H2SO4` is added while preparing a standard solution of Mohr's salt, we can break down the explanation into clear steps:

### Step-by-Step Solution:

1. **Understanding Mohr's Salt**:
- Mohr's salt is a double salt with the formula `NH4 whole twice FeSO4 whole twice 6H2O`, which can also be represented as `NH4Fe(SO4)2·6H2O`. It contains ferrous ions (Fe²⁺) and ammonium ions (NH4⁺).

2. **Identifying the Problem**:
- When Mohr's salt is dissolved in water, the ferrous ions (Fe²⁺) can undergo hydrolysis. Hydrolysis is a reaction with water that can lead to the formation of ferrous hydroxide (Fe(OH)₂), which is insoluble and can precipitate out of solution.

3. **Role of `H2SO4`**:
- To prevent hydrolysis of ferrous ions, concentrated sulfuric acid (`H2SO4`) is added. The addition of sulfuric acid helps maintain the acidity of the solution, which suppresses the hydrolysis reaction.

4. **Preventing Oxidation**:
- Another important reason for adding `H2SO4` is to prevent the oxidation of Fe²⁺ ions to Fe³⁺ ions. In an acidic environment, the oxidation of ferrous ions is minimized, ensuring that the ferrous state is maintained for accurate standardization.

5. **Conclusion**:
- Therefore, `H2SO4` is added while preparing a standard solution of Mohr's salt to prevent hydrolysis of ferrous ions and to prevent the oxidation of Fe²⁺ to Fe³⁺.

### Final Answer:
`H2SO4` is added while preparing a standard solution of Mohr's salt to prevent hydrolysis of ferrous ions and oxidation of Fe²⁺ to Fe³⁺.

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