Question

A solution of water (kf=1.86 ∘c/m) and glucose freezes at − 3.75 ∘c. What is the molal concentration of glucose in this solution? Assume that the freezing point of pure water is 0.00 ∘c.

Answer 1

Glucose freezes at -3.75 °C and freezing point of pure water is 0 °C. Thus, depression in freezing point or $\Delta T_{f}$ can be calculated as follows:

$\Delta T_{f}=0-(-3.75)^{o}C=3.75^{o}C$

It is related to molal concentration as follows:

$\Delta T_{f}=k_{f}m$

Here, $k_{f}$ is freezing point depression constant.

Rearranging to calculate molal concentration:

$m=\frac{\Delta T_{f}}{k_{f}}=\frac{3.75 ^{o}C}{1.86^{o}C/m}=2.02 m$

Therefore, molal concentration of glucose in the solution will be 2.02 m.