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. 


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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.

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