Q16 / 150 · MHT CET 2026 (16 April Shift 1) · Chemistry
For a binary solution, when a nonvolatile nonelectrolyte solute is dissolved in solvent, the vapour pressure of solvent decreases by 20 % . what is mole fraction of solvent in solution ?
- A$0.02$
- B$0.08$
- C$0.2$
- Correct answerD$0.8$
Solution
Let the vapour pressure of the pure solvent be $P^0$.
The decrease in vapour pressure is given as $20\%$, so the lowering of vapour pressure is $\Delta P = 0.20 P^0$.
According to Raoult's law, the relative lowering of vapour pressure is equal to the mole fraction of the solute, $X_{\text{solute}}$.
$\dfrac{\Delta P}{P^0} = X_{\text{solute}}$
$X_{\text{solute}} = \dfrac{0.20 P^0}{P^0} = 0.20$
For a binary solution, the sum of mole fractions of solute and solvent is $1$.
$X_{\text{solvent}} = 1 - X_{\text{solute}}$
$X_{\text{solvent}} = 1 - 0.20 = 0.8$
Answer: $0.8$
The decrease in vapour pressure is given as $20\%$, so the lowering of vapour pressure is $\Delta P = 0.20 P^0$.
According to Raoult's law, the relative lowering of vapour pressure is equal to the mole fraction of the solute, $X_{\text{solute}}$.
$\dfrac{\Delta P}{P^0} = X_{\text{solute}}$
$X_{\text{solute}} = \dfrac{0.20 P^0}{P^0} = 0.20$
For a binary solution, the sum of mole fractions of solute and solvent is $1$.
$X_{\text{solvent}} = 1 - X_{\text{solute}}$
$X_{\text{solvent}} = 1 - 0.20 = 0.8$
Answer: $0.8$
Hence, the correct answer is option: 'D'.