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Given vapour pressure (p°) = 121.8 mm; Weight of solute (w) = 15 g; Weight of solvent (W) = 250 g. From Raoult's law, p° - p / p° = w / m × M / W = 121.8 - 120.2 / 121.8 = 15 × 78 / 250 × 121.8 / 1.6 = 356.2.

1. p° - p / p° = w / m × M / W
2. 121.8 - 120.2 / 121.8
3. 15 × 78 / 250 × 121.8 / 1.6
4. 356.2

Correct answer: p° - p / p° = w / m × M / W

Solution

The question is based on Raoult's law, and the correct mathematical expression for the relative lowering of vapor pressure is given by option A: p° - p / p° = w / m × M / W.

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