Assertion (A) : The active mass of pure solid and pure liquid is taken unity.
Reason (R) : The active mass of pure solids and liquids depends on the density and molecular mass. The density and molecular of a mass of pure liquids and solids are constant.

Options

A. If both (A) and (R) are correct, and (R) is the correct explanation for (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.

Answer

To solve the given assertion and reason question, we need to analyze both the assertion (A) and the reason (R) to determine their validity and relationship.

### Step 1: Understand the Assertion (A)
The assertion states that "The active mass of pure solid and pure liquid is taken as unity."

- **Explanation**: In chemical equilibrium expressions, the active mass (or concentration) of pure solids and pure liquids is considered to be 1 (unity). This is because the concentration of a pure substance does not change during the reaction; it remains constant. Therefore, it is not included in the equilibrium constant expression.

### Step 2: Understand the Reason (R)
The reason states that "The active mass of pure solids and liquids depends on the density and molecular mass. The density and molecular mass of a mass of pure liquids and solids are constant."

- **Explanation**: The active mass is related to the concentration of the substance, which is defined as moles per unit volume. For pure solids and liquids, since their density and molecular mass are constant, the concentration remains constant throughout the reaction. Hence, we can say that the active mass does not vary and can be treated as unity.

### Step 3: Analyze the Relationship Between Assertion and Reason
- The assertion is true because we do take the active mass of pure solids and liquids as unity in equilibrium expressions.
- The reason is also true because it correctly explains why the active mass is considered constant (due to the constancy of density and molecular mass).

### Conclusion
Both the assertion (A) and the reason (R) are true, and the reason correctly explains the assertion. Therefore, the correct answer is that both A and R are true, and R is the correct explanation of A.

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

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