# A solution of urea (mol. mass 56 g mol-1) boils at 100.18°C at the atmospheric pressure. If Kf, and Kb, for water are 1.86 and 0.512 K kg kg mol-1 respectively, the above solution will freeze at

1. A

0.654°C

2. B

—0.654°C

3. C

654°C

4. D

—654°C

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### Solution:

if the value of m is same, then
Hence,  $\frac{{\mathrm{\Delta T}}_{\mathrm{f}}}{{\mathrm{K}}_{\mathrm{f}}}=\frac{{\mathrm{\Delta T}}_{\mathrm{b}}}{{\mathrm{K}}_{\mathrm{b}}}$
or ${\mathrm{\Delta T}}_{\mathrm{f}}={\mathrm{\Delta T}}_{\mathrm{b}}\frac{{\mathrm{K}}_{\mathrm{f}}}{{\mathrm{K}}_{\mathrm{b}}}$
$\left[{\mathrm{\Delta T}}_{\mathrm{b}}=100.18-100=0{.18}^{\circ }\mathrm{C}\right]$
$∆{\mathrm{T}}_{\mathrm{f}}=0.18×\frac{1.86}{0.512}=0{.65}^{\circ }\mathrm{C}$
As the Freezing Point of pure water is 0°C,

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