Question

$40$ g of a substance were dissolved in $450$ g of water to give a solution which freezes at $-{0.8}^{\circ }C$. Calculate molecular weight of substance. $K_f$ for water = $1.86$ K kg mol-1.

• A. $200.95$
• B. $206.67$
• C. $212.86$
• D. $264.95$

Answer 1

The correct option is B $206.67$

we have

$\Delta T_f=K_{f}m$

$0.8=1.86\ast \frac{40\ast 1000}{M\ast 450}$

we get Molecular weight = 206.6