Question

A solution of urea (molar mass 56 g/mol) boils at 100.18°C at the atmospheric pressure. If $K_f$ and $K_b$ of water are 1.86 and 0.52 K $kgmo{l}^{-1}$ respectively, the above solution will freeze at

• A. $-{0.654}^{\circ }C$
• B. ${0.654}^{\circ }C$
• C. ${6.54}^{\circ }C$
• D. $-{6.54}^{\circ }C$

Answer 1

The correct option is A

$-{0.654}^{\circ }C$

$\Delta T_f=k_f\times m$ and $\Delta T_b=k_b\times m$

$\frac{\Delta T_f}{\Delta T_b}=\frac{K_f}{K_b}$ and Given $\Delta T_b={0.18}^{\circ }C$

Hence $\Delta T_f=\frac{1.86}{0.512}\times 0.18=0.654$

$\therefore T_f=0-{0.654}^{\circ }C=-{0.654}^{\circ }C$