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The vapour pressure of acetone at 20 °C is 185 torr. When 1.2 g of a non-volatile substance was dissolved in 100 g of acetone at 20 °C, its vapour pressure was 183 torr. The molar mass (g mol−1) of the substance is:

1. 128
2. 488
3. 32
4. 64

Correct answer: 64

Solution

The decrease in vapour pressure due to the non-volatile solute can be calculated using Raoult's law, which relates the change in vapour pressure to the mole fraction of the solute. Given the data, the calculated molar mass of the solute is 64 g/mol, which matches the correct answer.

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