We’re being asked to determine the molar mass of an unknown solute that was dissolved in 50.0 g of water. The freezing point of the solution is –0.150 °C.
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)
Recall that the molality of a solution is given by:
For this problem, we need to do the following:
Step 1: Calculate for ΔTf.
Step 2: Determine the molality of the solution.
Step 3: Calculate the moles of the solute present.
Step 4: Calculate the molar mass of the solute.