Q.

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

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answer is B.

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Detailed Solution

Given, po = 185 torr at 20oC and ps - 183 torr at 20oC Mass of non-volatile substance, m - 1.2 g Mass of acetone taken = 100 g and M = ?As, we have po-psps=nNPutting the values, we get, 185-183183=12M100 ⇒2183=1.2×58100×M ∴ M=183×1.2×582×100=63.684 g ≈64 g mol-1

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