1.1 What is Chemistry?

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

ØA=nntotalA

Note that we are dealing with the total number of particles (or moles of particles), so we need to take the dissociation
of ionic substances into account. If we dissolve one mole of NaCl in water, we would end up with two moles
of independent particles (one mole of sodium ions and one mole of chloride ions). This information needs to be
included in any mole fraction calculations.

Example 16.13

22.6 grams of KCl is dissolved in 2000. grams of water. Determine the mole fraction of KCl.

Answer:

ØA=

nA

ntotal

nA= 22 .6 g KCl×

1 mol KCl

74 .55 g KCl

×

2 mol ions

1 mol KCl

= 0 .606 mol ions

nH 2 O= 2000 .g H 2 O×

1 mol H 2 O

18 .02 g H 2 O

= 111 .0 mol H 2 O

ntotal= 0 .606 mol+ 111 .0 mol= 111 .6 mol

ØA=

0 .606 mol

111 .6 mol

= 0. 00543

Boiling Point Elevation

Recall that boiling occurs when the vapor pressure of a liquid is equal to the atmospheric pressure. Since adding
a solute lowers the vapor pressure, we would expect a higher temperature to be required before boiling can begin.
This phenomenon, known asboiling point elevation, occurs whenever a solute is dissolved into a pure solvent.

FIGURE 16.14