The mole fraction of the solvent in the solution of a non-volatile solute is 0.980. The relative lowering of vapour pressure is :

Options

A. 0.01
B. 0.98
C. 0.02
D. 0.49

Answer

To solve the problem of finding the relative lowering of vapor pressure when the mole fraction of the solvent is given, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept**: The relative lowering of vapor pressure is given by the formula:
\[ \text{Relative lowering of vapor pressure} = \frac{P_0 - P}{P_0} = \text{mole fraction of solute} \]
where \( P_0 \) is the vapor pressure of the pure solvent and \( P \) is the vapor pressure of the solution.

2. **Identify Given Information**: We are given the mole fraction of the solvent in the solution, which is:
\[ \text{Mole fraction of solvent} = 0.980 \]

3. **Calculate the Mole Fraction of Solute**: The sum of the mole fractions of the solute and solvent is equal to 1. Therefore, we can find the mole fraction of the solute using the formula:
\[ \text{Mole fraction of solute} = 1 - \text{Mole fraction of solvent} \]
Substituting the given value:
\[ \text{Mole fraction of solute} = 1 - 0.980 = 0.020 \]

4. **Determine the Relative Lowering of Vapor Pressure**: From the formula stated in the first step, we know that the relative lowering of vapor pressure is equal to the mole fraction of the solute:
\[ \text{Relative lowering of vapor pressure} = \text{Mole fraction of solute} = 0.020 \]

5. **Final Answer**: Thus, the relative lowering of vapor pressure is:
\[ \text{Relative lowering of vapor pressure} = 0.020 \]

### Summary:
The relative lowering of vapor pressure when the mole fraction of the solvent is 0.980 is 0.020.