Question

When a solution containing $wg$ of urea in $1kg$ of water is cooled to $-{0.372}^{\circ }C,200g$ of ice is separated. If $K_f$ for water is $1.86Kkgmo{l}^{-1},w$ is:

• A. $4.8g$
• B. $12.0g$
• C. $9.6g$
• D. $6.0g$

Answer 1

The correct option is C $9.6g$

Molar mass of urea $\left(m\right)=60g$
Mass of water present in the solution at the freezing point

$=1000-200=800g$
Depression in freezing point is given by:

$\Delta T_f=0.372=\frac{1000\times K_f\times w}{60\times 800}=\frac{1000\times 1.86\times w}{60\times 800}$

$orw=9.6g$
$\therefore$ Amount of urea added = $9.6g$