Assertion : Vapour pressure of solvent increases when solvent B is added.
Reason : B is more volatile therefor vapour pressure of B is greater than of A.

Answer

To solve the given question, we will analyze both the assertion and the reason step by step.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that the vapor pressure of solvent increases when solvent B is added. This implies that the addition of solvent B has a positive effect on the vapor pressure of the solution.

2. **Understanding the Reason**:
- The reason provided is that solvent B is more volatile than solvent A, which suggests that the vapor pressure of B is greater than that of A.

3. **Analyzing Vapor Pressure**:
- Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid phase. When two volatile solvents are mixed, the total vapor pressure of the solution is the sum of the vapor pressures of the individual components:
\[ P_{\text{total}} = P_A + P_B \]
- Here, \(P_A\) is the vapor pressure of solvent A and \(P_B\) is the vapor pressure of solvent B.

4. **Effect of Adding Solvent B**:
- If solvent B is added and is volatile, it will contribute to the total vapor pressure. Therefore, the assertion that the vapor pressure increases when B is added is correct.

5. **Evaluating the Reason**:
- The reason states that B is more volatile, which implies that its vapor pressure is greater than that of A. However, the assertion does not require B to be more volatile; it only states that the vapor pressure increases. Even if B is less volatile, its presence still contributes to the total vapor pressure.
- Thus, while B being more volatile could lead to a greater increase in vapor pressure, it is not necessary for the assertion to be true.

6. **Conclusion**:
- The assertion is true: the vapor pressure of the solvent increases when solvent B is added.
- The reason is false: it is not necessary for B to be more volatile for the assertion to hold true.

### Final Answer:
- **Assertion**: True
- **Reason**: False