Which of the following statements are true for Mohr's salt?

Options

A. It decolourizes `KMnO_(4)` solution
B. It is a double salt
C. It is colourless salt
D. It is a primary standard substance

Answer

To determine which statements are true for Mohr's salt, we will analyze each statement based on the properties and characteristics of Mohr's salt.

### Step-by-Step Solution:

1. **Understanding Mohr's Salt**:
- Mohr's salt is a double salt composed of ferrous ammonium sulfate, represented as \( \text{FeSO}_4 \cdot \text{(NH}_4\text{)}_2\text{SO}_4 \cdot 6\text{H}_2\text{O} \). It contains both ferrous ions (\( \text{Fe}^{2+} \)) and ammonium ions (\( \text{NH}_4^+ \)).

2. **Evaluating Statement A**:
- **Statement**: "It decolorizes KMnO4 solution."
- **Analysis**: Mohr's salt is used in redox titrations where it reduces potassium permanganate (\( \text{KMnO}_4 \)) to manganese ions (\( \text{Mn}^{2+} \)), causing the purple color of \( \text{KMnO}_4 \) to disappear (decolorization).
- **Conclusion**: This statement is **true**.

3. **Evaluating Statement B**:
- **Statement**: "It is a double salt."
- **Analysis**: Mohr's salt is indeed a double salt because it retains its identity in the crystal lattice and dissociates into its constituent ions when dissolved in water.
- **Conclusion**: This statement is **true**.

4. **Evaluating Statement C**:
- **Statement**: "It is a colorless salt."
- **Analysis**: Mohr's salt is not colorless; it is typically blueish-green in color due to the presence of ferrous ions.
- **Conclusion**: This statement is **false**.

5. **Evaluating Statement D**:
- **Statement**: "It is a primary standard substance."
- **Analysis**: Mohr's salt is used as a primary standard in redox titrations, particularly with \( \text{KMnO}_4 \), due to its stable composition and known concentration.
- **Conclusion**: This statement is **true**.

### Final Conclusion:
The true statements about Mohr's salt are:
- A: It decolorizes KMnO4 solution. (True)
- B: It is a double salt. (True)
- C: It is a colorless salt. (False)
- D: It is a primary standard substance. (True)

Thus, the correct options are A, B, and D.