Question

The vapour pressure of water at room temperature is lower by $5\%$ by dissolving a solute in it, then the approximately molality of solution is

• A. $2$
• B. $1$
• C. $4$
• D. $3$

Answer 1

The correct option is D $3$

$\frac{P^{\circ }-P}{P^{\circ }}=0.05$
$\therefore \frac{P^{\circ }-P}{P^{\circ }}=\frac{1}{\frac{1}{0.05}-1}=0.0526=\frac{n}{N}$
$\text{Molality}=\frac{0.0526\times 1000}{18}=2.92$