Problem: When 1.38 g of an unknown non-electrolyte is dissolved in 50.0 g of acetone, the freezing point decreased by 0.52 degrees C. If the Kfp of the solvent is 2.4 K/m, calculate the molar mass of the unknown solute.

FREE Expert Solution

We’re being asked to calculate the molar mass of the unknown solute.


Recall that the freezing point of a solution is lower than that of the pure solvent and the change in freezing point (ΔT­f) is given by:


ΔTf=Tf, pure solvent-Tf, solution


The change in freezing point is also related to the molality of the solution:


ΔTf=imKf


where:

i = van’t Hoff factor

m = molality of the solution (in m or mol/kg)

Kf = freezing point depression constant (in ˚C/m) of the solvent


We go through the following steps to solve the problem: 

Step 1. Calculate the molality of the solution

Step 2. Calculate the moles solute

Step 3. Calculate the molar mass of the solute. 


View Complete Written Solution
Problem Details

When 1.38 g of an unknown non-electrolyte is dissolved in 50.0 g of acetone, the freezing point decreased by 0.52 degrees C. If the Kfp of the solvent is 2.4 K/m, calculate the molar mass of the unknown solute.