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:

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

$\overline{){{\mathbf{\Delta T}}}_{{\mathbf{f}}}{\mathbf{=}}{{\mathbf{imK}}}_{{\mathbf{f}}}}$

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:

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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