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 Given Information**:
- The mole fraction of the solvent (X1) is given as 0.980.
- We need to find the relative lowering of vapor pressure.

2. **Use the Relationship of Mole Fractions**:
- The sum of the mole fractions of the solvent (X1) and solute (X2) is equal to 1:
\[ X1 + X2 = 1 \]
- Substituting the value of X1:
\[ 0.980 + X2 = 1 \]

3. **Solve for the Mole Fraction of the Solute (X2)**:
- Rearranging the equation to find X2:
\[ X2 = 1 - 0.980 = 0.020 \]

4. **Apply the Formula for Relative Lowering of Vapor Pressure**:
- The relative lowering of vapor pressure is equal to the mole fraction of the solute:
\[ \text{Relative lowering of vapor pressure} = X2 \]
- Thus, we have:
\[ \text{Relative lowering of vapor pressure} = 0.020 \]

5. **Identify the Correct Option**:
- The calculated relative lowering of vapor pressure is 0.020, which corresponds to option 3.

### Final Answer:
The relative lowering of vapor pressure is **0.02**.

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