Question

A solution of urea in water has boiling point of $100.15^{\circ}C$. Calculate the freezing point of the same solution if $K_{f}$ and $K_{b}$ for water are $1.87 \,K \,kg\, mol^{-1}$ and $0.52 \,K\, kg \,mol^{-1}$ respectively

• A. $- 0.54^{\circ}C$
• B. $- 0.44^{\circ}C$
• C. $- 0.64^{\circ}C$
• D. $- 0.34^{\circ}C$

Answer 1

Answer: (A)

$\Delta T_{b}=(100.15-100)=0.15^{\circ}C$
We know that , $\Delta\,T_{b}$ = molality $\times K_{b}$
Molality $=\frac{\Delta\,T_{b}}{K_{b}}=\frac{0.15}{0.52}$
$=0.2884$
$\Delta\,T_{f}$ = molality $\times K_{f}$
$=0.2884 \times 1.87=0.54^{\circ}C$
Thus, the freezing point of the solution
$=-0.54^{\circ}C$