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The vapour pressure at a given temperature of an ideal solution containing 0.2 mol of a non-volatile solute and 0.8 mol of solvent is 60 mm of Hg. The vapour pressure of the pure solvent at the same temperature is

1. 75 mm Hg
2. 60 mm Hg
3. 80 mm Hg
4. 90 mm Hg

Correct answer: 90 mm Hg

Solution

The vapour pressure of the solution is given by Raoult's law: P_solution = X_solvent × P_pure_solvent. Here, X_solvent = 0.8 / (0.8 + 0.2) = 0.8. Rearranging, P_pure_solvent = P_solution / X_solvent = 60 / 0.8 = 75 mm Hg.

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