Question

Freezing point of a $5\%$ aqueous solution of $X$ is equal to freezing point of $20\%$ aqueous solution of $Y$. If molecular weight of $X$ is $A$, then molecular weight of $Y$ is:

• A. 4.75A
• B. A
• C. 4A
• D. 2A

Answer 1

The correct option is A 4.75A

For same freezing point, molality of both solution should be same.
$m_x=m_y$
$m_x=\frac{\text{weight of x}\times 1000}{\text{molecular weight (Mwt) of}\times \left(M_x\right)\times \text{weight of solvent}}$
$m_y=\frac{\text{weight of y}\times 1000}{M_y\times \text{weight of solvent}}$
So, $\frac{5\times 1000}{95\times M_x}=\frac{20\times 1000}{80\times M_y}$ or,
$M_y=\frac{95\times 20}{5\times 80}{M}_x=4.75$ A
Closest option is 4.75A.