The Foundations of Chemistry

Mole Fraction

Recall that in Chapter 12 the mole fractions,XAand XB, of each component in a mixture

containing components A and B were defined as

XA and XB

Mole fraction is a dimensionless quantity, that is, it has no units.

EXAMPLE 14-3 Mole Fraction

What are the mole fractions of CH 3 OH and H 2 O in the solution described in Example 14-1?

It contains 128 grams of CH 3 OH and 108 grams of H 2 O.

Plan

We express the amount of both components in moles, and then apply the definition of mole

fraction.

Solution

_?_ mol CH 3 OH128 g CH 3 OH4.00 mol CH 3 OH

_?_ mol H 2 O108 g H 2 O6.00 mol H 2 O

Now we calculate the mole fraction of each component.

XCH 3 OH0.400

XH 2 O0.600

You should now work Exercise 30.

COLLIGATIVE PROPERTIES OF SOLUTIONS

Physicalproperties of solutions that depend on the number,but not the kind,of solute parti-

cles in a given amount of solvent are called colligative properties.There are four

important colligative properties of a solution that are directly proportional to the number

of solute particles present. They are (1) vapor pressure lowering, (2) boiling point eleva-

tion, (3) freezing point depression, and (4) osmotic pressure. These properties of a solution

depend on the total concentration of all solute particles,regardless of their ionic or molecular

nature, charge, or size. For most of this chapter, we will consider nonelectrolytesolutes

(Section 4-2, part 1); these substances dissolve to give one mole of dissolved particles for

each mole of solute. In Section 14-14 we will learn to modify our predictions of colliga-

tive properties to account for ion formation in electrolyte solutions.

6.00 mol



(4.00
6.00) mol

no. mol H 2 O



no. mol CH 3 OH
no. mol H 2 O

4.00 mol



(4.00
6.00) mol

no. mol CH 3 OH



no. mol CH 3 OH
no. mol H 2 O

1 mol H 2 O



18.0 g H 2 O

1 mol CH 3 OH



32.0 g CH 3 OH

no. mol B



no. mol A
no. mol B

no. mol A



no. mol A
no. mol. B

In any mixture the sum of the mole
fractions must be 1:

0.400
0.600 1

556 CHAPTER 14: Solutions

Colligativemeans “tied together.”