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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. 32
2. 64
3. 128
4. 488

Correct answer: 64

Solution

The decrease in vapor pressure of acetone upon dissolving the non-volatile substance can be calculated using Raoult's law. The change in vapor pressure indicates the number of moles of solute present, and given the mass of the solute and the mass of the solvent, the molar mass can be determined, leading to the conclusion that the correct answer is 64 g/mol.

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