Question

$3g$ urea is dissolved in $45g$ of water. The relative lowering of vapour pressure is :

• A. $0.05$
• B. $0.04$
• C. $0.02$
• D. $0.01$

Answer 1

The correct option is C $0.02$

$\text{Mass of urea given}\left(N{H}_{2}CON{H}_2\right)=3g$
$\text{Moles of urea}=\frac{3}{60}=0.05mol$

number of moles of the solute is negibile compared to the moles of solvent.
The relative lowering of vapour pressure is given by,

$\frac{\Delta P}{P_0}={\chi }_B=\frac{0.05}{\frac{45}{18}+0.05}=0.0196\approx 0.02$