Question

4.0 g of substance A dissolved in 100 g $H_{2}O$ depressed the freezing point of water by ${0.1}^{o}C$ while 4.0 g of another substance B depressed the freezing point by ${0.2}^{o}C$. Which one has higher molecular mass and what is the relation?

• A. $M_A=M_B$
• B. $M_A=2M_B$
• C. $M_A=3M_B$
• D. $M_A=1.5M_B$

Answer 1

The correct option is B $M_A=2M_B$

we have,

$\Delta T_f=K_{f}m$

for substance A

$0.1=K_f\times \frac{4g}{M_A\times 100g}\times 1000$

$M_A=400K_f$

for substance B,

$0.2=K_f\times \frac{4g}{M_B\times 100g}\times 1000$

$M_B=200K_f$

We get, $M_A=2M_B$

Hence, option (B) is correct.