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Question

The vapour pressure of acetone at 20C is 185 torr. When 1.2 g of a non-volatile substance was dissolved in 100 g of acetone at 20C, its vapour pressure was 183 torr.

The molar mass (gmol1) of the substance is:

A
32
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B
64
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C
128
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D
488
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Solution

The correct option is B 64
The molar mass M2 of non-volatile substance can be calculated from the following formula.

p0pp0=W2×M1W1×M2

Here, p0 vapour pressure of acetone and p is the vapour pressure of the solution.
M1 and W1 are the molar mass and mass of acetone.
M2 and W2 are the molar mass and mass of a non-volatile substance.

Substitute values in the above expression.

185183185=1.2×58100×M2

Hence, M2=64.38g/mol.
Thus, the molar mass of the non-volatile substance is 64 g/mol (approximately).
Hence, the correct option is B

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