5 Steps to a 5 AP Chemistry 2019

186 ❯ STEP 4. Review the Knowledge You Need to Score High

Let’s learn to apply the preceding equation. Determine the freezing point of an aque-

ous solution containing 10.50 g of magnesium bromide in 200.0 g of water.

∆= =





































=

=−==−°

TiKm −

T

3(1.86Kkgmol )

(10.50gMgBr)

(200.0g)

(1 moleMgBr )

184.113gMgBr

1 kg

1, 000 g

1.59 K

(273.15 1.59)K 271.56K( 1.59C)

f

1 2

2

2

fp

The most common mistake is to forget to subtract the DT value from the normal freez-

ing point.

The freezing-point depression technique is also commonly used to calculate the

molar mass of a solute.

For example, a solution is prepared by dissolving 0.490 g of an unknown compound

in 50.00 mL of water. The freezing point of the solution is –0.201°C. Assuming

the compound is a nonelectrolyte, what is the molecular mass of the compound? Use

1.00 g/mL as the density of water.

m = DT/Kf = 0.201 K/(1.86 K kg mol-^1 ) = 0.108 mol/kg

50.00 mL (1.00 g/mL) (1 kg/1,000 g) = 0.0500 kg

(0.108 mol/kg) (0.0500 kg) = 0.00540 mol

0.490 g/0.00540 mol = 90.7 g/mol

Many students make the mistake of stopping before they complete this problem.

Boiling-Point Elevation

Just as the freezing point of a solution of a nonvolatile solute is always lower than

that of the pure solvent, the boiling point of a solution is always higher than the

solvent’s. Again, only the number of solute particles affects the boiling point. The

mathematical relationship is similar to the one for the freezing-point depression

above and is

DTb = iKb molality

where DTb is the number of degrees the boiling point has been elevated (the difference

between the boiling point of the pure solvent and the solution); Kb is the boiling-point

elevation constant; the molality is the molality of the solute; and i is the van’t Hoff

factor. You can calculate a solution’s boiling point if you know the molality of the solu-

tion. If you know the amount of the boiling-point elevation and the molality of the

solution, you can calculate the value of the van’t Hoff factor, i.