Question

At ${20}^{\circ }\text{C}$ The vapour pressure of pure ether is $442\text{mm Hg}$. When $6.1\text{g}$ of a non volatile substance is dissolved in $50\text{g}$ of ether (molecular weight $74$). The vapour pressure of solution falls to $410\text{mm Hg}$.
What is the molecular weight of the substance?
Assume the solution to be non-dilute.

• A. $74.2\text{g/mol}$
• B. $115.6\text{g/mol}$
• C. $33.9\text{g/mol}$
• D. $50.5\text{g/mol}$

Answer 1

The correct option is B $115.6\text{g/mol}$

For a non-dilute solution:

$\frac{n_{solute}}{n_{solvent}}=\frac{P_{solvent}^{\circ }-P_{solution}}{P_{solution}}=\frac{442-410}{410}$

$\Rightarrow \frac{\frac{6.1}{M_{solute}}}{\frac{50}{74}}=\frac{32}{410}$

$\Rightarrow M_{solute}=\frac{6.1\times 74\times 410}{50\times 32}=115.67\text{g/mol}$