Assertion `(A)`: Hardness of water is determined by titrating it with disodium salt of `EDTA`.
Reason `(R)` :The indicator used in the titration is Eriochrome Black`-T` at `pH=10`.

Options

A. If both (A) and (R) are correct and (R) is the correct explanation of (A).
B. If both (A) and (R) are correct but (R) is not the correct explanation of (A).
C. If (A) is correct, but (R) is incorrect.
D. If (A) is incorrect, but (R) is correct.

Explanation

### Step-by-Step Solution:

1. **Understanding Hardness of Water**:
- Hardness of water is primarily due to the presence of calcium (Ca²⁺) and magnesium (Mg²⁺) ions. These ions can form insoluble salts with soap, which is why hard water does not lather well.

2. **Titration with EDTA**:
- The hardness of water can be determined by titrating it with a chelating agent known as EDTA (ethylenediaminetetraacetic acid). EDTA forms stable complexes with calcium and magnesium ions, allowing for their quantification.

3. **Disodium Salt of EDTA**:
- The disodium salt of EDTA is commonly used in this titration. It is represented as Na₂[EDTA], which effectively binds to the metal ions present in the water sample.

4. **Role of Indicator**:
- In this titration, an indicator called Eriochrome Black T is used. This indicator changes color when it binds to calcium ions, forming a complex that is wine-red in color.

5. **pH Consideration**:
- The titration is typically conducted at a pH of around 10. At this pH, the calcium ions form a complex with the indicator, and when EDTA is added, it displaces the indicator from the complex, resulting in a color change from wine-red to blue.

6. **Conclusion**:
- Both the assertion (A) that hardness of water is determined by titrating it with the disodium salt of EDTA, and the reason (R) that the indicator used is Eriochrome Black T at pH 10 are true. Additionally, the reason correctly explains the assertion.

### Final Answer:
- **Both Assertion (A) and Reason (R) are correct, and R is the correct explanation of A**.

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