What is the freezing point of a 0.400 m aqueous solution of a nonvolatile nonelectrolyte? ( The freezing point depression constant for water is 1.86⁰ C/m) A)0.744^° C B)0.00^° C C)-7.44^° C D)-0.744^° C

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Quizplus · Accepted Answer

The freezing point depression equation is ΔT = Kf*m, where ΔT is the change in freezing point, Kf is the freezing point depression constant, and m is the molality of the solution (mol solute/kg solvent). 
Since the solute is a nonvolatile nonelectrolyte, it does not dissociate into ions and its concentration in molality is equal to its concentration in molarity. 
Assuming a density of 1 g/mL for water, we have 400 g of solution, which is equal to 0.4 kg. 
Since the solute is nonvolatile, its vapor pressure does not contribute to the total vapor pressure of the solution, so we can assume that the solvent is still water. 
Therefore, 
m = 0.400 m 
ΔT = Kf*m = (1.86 °C/m)(0.400 m) = 0.744 °C 
Since the freezing point depression is a lowering of the freezing point, the actual freezing point of the solution will be less than the normal freezing point of pure water (0 °C). 
Therefore, the answer is D, -0.744 °C.

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