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According to Raoult's law, p° - p / p° = n / n + N (mole fraction of solute). For other solution of same solvent, p° - p / p° = n / n + N (mole fraction of solute). Mole fraction of solvent = 1 - 0.4 = 0.6. Decrease in vapour pressure μ Mole fraction of solute. Thus, depression of 2 × 0.2 mm Hg vapour pressure requires 2 × 0.2 mole fraction of solute.

1. p° - p / p° = n / n + N
2. p° = 50 mm
3. Mole fraction of solute = 0.4
4. Depression of 2 × 0.2 mm Hg vapour pressure requires 2 × 0.2 mole fraction of solute.

Correct answer: p° - p / p° = n / n + N

Solution

Raoult's law states that the relative lowering of vapor pressure is directly proportional to the mole fraction of the solute, which is expressed as p° - p / p° = n / (n + N). This matches option A.

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