Question

The vapour pressure of acetone at ${20}^{\circ }C$ is 185 torr. When 1.2 g of a non-volatile substance was dissolved in 100 g of acetone at ${20}^{\circ }C$, its vapour pressure was 183 torr. The molar mass $\left(gmo{l}^{-1}\right)$ of the substance is:

• A. 32
• B. 64
• C. 128
• D. 488

Answer 1

The correct option is B

64

$\frac{P^{\circ }-P}{P^{\circ }}=X_{solute}=\frac{n_1}{n_1+n_2}\frac{185-183}{185}=\frac{2}{185}=\frac{\frac{1.2}{M}}{\frac{1.2}{M}+\frac{100}{58}}\Rightarrow M=64$