Question

Calculate the relative lowering of vapour pressure when $12g$ urea (Mol weight 60 g/mol) is dissolved in $90g$ of water.

• A. $0.011$
• B. $0.11$
• C. $0.038$
• D. $0.38$

Answer 1

The correct option is C $0.038$

Relative lowering of vapour pressure:
$\frac{\Delta P}{P_A^{\circ }}=\frac{n_B}{n_A+n_B}$
$n_B=\text{Moles of urea}$
$n_A=\text{Moles of water}$
$n_B=\frac{12}{60}=0.2;n_A=\frac{90}{18}=5$

$\frac{△p}{p_0}=\frac{0.2}{5+0.2}=0.038$