Question

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

• A. $74.2g$
• B. $33.9g$
• C. $115.6g$
• D. $50.5g$

Answer 1

The correct option is C $115.6g$

For a non-dilute solution:

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

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

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