For a dilute solution,
lowering of vapour pressure `prop` mole fraction of the solute
or, lowering of vapour pressure = K `xx` mole fraction of the solute, where K is

Options

A. a constant for the solute
B. a constant for the solvent
C. a constant for the solution
D. vapour pressure of the solvent

Answer

To solve the question regarding the relationship between the lowering of vapor pressure and the mole fraction of the solute in a dilute solution, we can break it down step by step.

### Step-by-Step Solution:

1. **Understanding the Concept**:
- In a dilute solution, the vapor pressure of the solution is lower than that of the pure solvent. This phenomenon is known as the lowering of vapor pressure.

2. **Establishing the Relationship**:
- According to Raoult's Law, the lowering of vapor pressure (ΔP) is directly proportional to the mole fraction of the solute (X_solute). This can be expressed mathematically as:
\[ \Delta P = K \cdot X_{\text{solute}} \]
- Here, K is a proportionality constant.

3. **Identifying K**:
- The constant K in this equation is related to the vapor pressure of the pure solvent. Specifically, it can be defined as:
\[ K = P^0_{\text{solvent}} \]
- Where \( P^0_{\text{solvent}} \) is the vapor pressure of the pure solvent.

4. **Final Equation**:
- Therefore, we can rewrite the equation for lowering of vapor pressure as:
\[ \Delta P = P^0_{\text{solvent}} \cdot X_{\text{solute}} \]

5. **Conclusion**:
- The lowering of vapor pressure is proportional to the mole fraction of the solute, and K is the vapor pressure of the pure solvent.

### Answer:
K is the vapor pressure of the solvent.