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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

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