A: Lowering of vapour pressure depends upon concentration of solute.
R: Relative lowering of vapour pressure is a colligative property

Answer

To solve the question, we need to evaluate both the assertion and the reason provided.

### Step 1: Analyze the Assertion
The assertion states that "Lowering of vapour pressure depends upon concentration of solute."

- **Explanation**: When a non-volatile solute is added to a solvent, the vapour pressure of the solvent decreases. This is because the presence of solute particles reduces the number of solvent molecules that can escape into the vapour phase. The extent of this lowering is directly related to the concentration of the solute in the solution.

### Step 2: Analyze the Reason
The reason states that "Relative lowering of vapour pressure is a colligative property."

- **Explanation**: Colligative properties are properties that depend on the number of solute particles in a solution, not on the nature of the solute. The relative lowering of vapour pressure is indeed a colligative property because it depends on the ratio of the number of solute particles to the total number of particles in the solution.

### Step 3: Determine the Truth of Assertion and Reason
- Both the assertion and the reason are true.
- The reason correctly explains the assertion, as the dependence of vapour pressure lowering on the concentration of solute is a characteristic of colligative properties.

### Conclusion
Both the assertion and the reason are true, and the reason is the correct explanation of the assertion.

**Final Answer**: Both assertion and reason are true, and the reason is the correct explanation of the assertion.

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