We’re being asked to determine the molar mass of an unknown of non-electrolytic solute present in a solution with a freezing point of –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 unknown solute present.
Step 4: Calculate the molar mass of the solute.
0.528 grams of an unknown non-electrolytic solute were dissolved in 50.0 g of water (Kfp = 1.86 K/m). If the freezing point of the solution was -0.150 °C, determine the molar mass of the unknown solute.
A. 1174 g/mol
B. 0.85 g/mol
C. 0.13 g/mol
D. 131 g/mol
E. 95 g/mol
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