Question

When $3g$ urea (Mol weight 60 g/mol) is dissolved in $45g$ of water. The relative lowering of vapour pressure is:

• A. $0.18$
• B. $0.12$
• C. $0.08$
• D. $0.02$

Answer 1

The correct option is D $0.02$

Relative lowering of vapour pressure:
$\frac{\Delta P}{P_A^{\circ }}=\frac{n_B}{n_A+n_B}$
$n_B=\text{Moles of urea}$
$n_A=\text{Moles of water}$
$n_B=\frac{3}{60}=0.05;n_A=\frac{45}{18}=2.5$

$\frac{△p}{p_0}==\frac{0.05}{2.5+0.05}=0.0196\approx 0.02$