We’re being asked to determine the volume of ethanol (C2H5OH) that must be added to 450 mL water to get a solution with a freezing point of –15.0 ˚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 volume of C2H5OH needed.
What volume of ethanol (density = 0.7893 g/cm3) should be added to 450 mL of water in order to have a solution that freezes at -15.0°C? [For water, Kf = 1.86°C/m.]
A) 371 mL
B) 470 mL
C) 212 mL
D) 132 mL
E) 167 mL
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