Question 70

When a non-volatile solute is added to the solvent, the vapour pressure of the solvent decreases by 10 mm of Hg . The mole fraction of the solute in the solution is 0.2 . What would be the mole fraction of the solvent if decrease in vapour pressure is 20 mm of Hg ?

Vapor pressure decreases by 10 mm Hg when mole fraction of solute is 0.2. Find mole fraction of solvent when decrease is 20 mm Hg.

Apply Raoult's law for relative lowering

$$\frac{\Delta P}{P°} = x_{solute}$$

$$\frac{10}{P°} = 0.2 \Rightarrow P° = 50$$ mm Hg

For decrease of 20 mm Hg

$$x_{solute} = \frac{20}{50} = 0.4$$

$$x_{solvent} = 1 - 0.4 = 0.6$$

The correct answer is Option 4: 0.6.

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