CK-12-Chemistry Intermediate

16.3. Colligative Properties http://www.ck12.org

FIGURE 16.13

The lowering of the vapor pressure in a

solution causes the boiling point of the

solution to be higher than that of the pure

solvent.

The magnitude of the boiling point elevation is also directly proportional to the molality of the solution. The equation
is:

∆Tb= Kb×m

The proportionality constant, Kb, is called themolal boiling-point elevation constant.It is a constant that is equal
to the change in the boiling point for a 1-molal solution of a nonvolatile molecular solute. For water, the value of
Kbis 0.512 °C/m. The boiling temperature of a 1-molal aqueous solution of any nonvolatile molecular solute is
100.512°C.

Electrolytes and Colligative Properties

As discussed earlier, ionic compounds are electrolytes that dissociate into two or more ions as they dissolve. This
must be taken into account when calculating the freezing and boiling points of electrolyte solutions. Sample Problem
16.7 demonstrates how to calculate the freezing point and boiling point of a solution of calcium chloride. Calcium
chloride dissociates into three ions according to the following equation:

CaCl 2 (s)→Ca^2 +(aq) + 2Cl−(aq)

The values of the freezing point depression and the boiling point elevation for a solution of CaCl 2 will be three times
greater than they would be for an equal molality of a nonelectrolyte.

Sample Problem 16.7: Freezing and Boiling Point of an Electrolyte

Determine the freezing and boiling point of a solution prepared by dissolving 82.20 g of calcium chloride in 400. g
of water.

Step 1: List the known quantities and plan the problem.

Known