Chemistry Practice Problems Freezing Point Depression Practice Problems Solution: How many grams of ethylene glycol, C 2H4(OH)2, mus...

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Solution: How many grams of ethylene glycol, C 2H4(OH)2, must be added to 400.0 g of water to yield a solution that will freeze at -8.35 °C?a) 37 gb) 111 gc) 75 gd) 151 g

Problem

How many grams of ethylene glycol, C 2H4(OH)2, must be added to 400.0 g of water to yield a solution that will freeze at -8.35 °C?

a) 37 g

b) 111 g

c) 75 g

d) 151 g


Solution

We’re being asked to determine the mass of ethylene glycol (C2H4(OH)2) that must be added to 400.0 g water to get a solution with a freezing point of –8.35 ˚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:


ΔTf=Tf, pure solvent-Tf, solution


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


ΔTf=imKf


where: 

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:


molality=moles solutekg solvent



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 mass of C2H4(OH)2 needed.


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