Question

A solution of urea (mol. mass 56g mol^–1) boils at 100.18°C at the atmospheric pressure. If $K_{f}$ and $K_{b}$for water are 1.86 and 0.512K kg mol^–1 respectively the above solution will freeze at.

• A. – 6.54°C
• B. 6.54°C
• C. 0.654°C
• D. –0.654°C

Answer 1

Answer: (D)

–0.654°C

Answer 2

$\Delta T_{b} = 0.18$; $\Delta T_{b} = mK_{b}$

$\frac{0.18}{\Delta T_{f}} = \frac{mK_{b}}{mK_{f}}$; $\frac{0.18 \times 1.86}{0.512} = \Delta T_{f}$ ; $\Delta T_{f} = 0.653$

$T^{0} - T_{s} = 0.653$;$T^{0} - T_{s} = 0.653$; $T_{s} = 0 - 0.653^{o}C$.