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 $\left(g/mol\right)$ of the substance is:

• A. 32
• B. 64
• C. 128
• D. 488

Answer 1

The correct option is B 64

We know that,
$\frac{P_o-P_s}{P_s}=\frac{n}{N}$
$\frac{185-183}{183}$ = $\frac{\frac{1.2}{M}}{\frac{100}{58}}$

$M\approx 64g/mol$