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