The Foundations of Chemistry

570 CHAPTER 14: Solutions

Plan for (b)

The percent ionization is given by

% ionization100% (where moriginalmstated0.0100 m)

The freezing point depression is caused by the meffective, the total concentration of all dissolved

species—in this case, the sum of the concentrations of HA, H
, and A. We know the value

of meffectivefrom part (a). Thus, we need to construct an expression for the effective molality

in terms of the amount of lactic acid that ionizes. We represent the molality of lactic acid

that ionizes as an unknown, x,and write the concentrations of all species in terms of this

unknown.

Solution for (b)

In many calculations, it is helpful to write down (1) the values, or symbols for the values,

of initial concentrations; (2) changes in concentrations due to reaction; and (3) final concen-

trations, as shown here. The coefficients of the equation are all ones, so the reaction ratio

must be 1
 1 
1.

Let xmolality of lactic acid that ionizes; then

xmolality of H
and lactate ions that have been formed

HA 88n H
 
 A

Start 0.0100 m 00

Change x m 
x m 
x m

Final (0.0100x)mx mx m

The meffectiveis equal to the sum of the molalities of all the solute particles.

meffectivemHA 
mH

mA

(0.0100x) m
x m 
x m(0.0100
x) m

This must equal the value for meffectivecalculated earlier, 0.0111 m.

0.0111 m(0.0100
x) m

x0.0011 mmolality of the acid that ionizes

We can now calculate the percent ionization.

% ionization100%100% 11%

This experiment shows that in 0.0100 msolutions, only 11% of the lactic acid has been

converted into H
and C 2 H 4 (OH)COOions. The remainder, 89%, exists as nonionized

molecules.

You should now work Exercises 78 and 80.

0.0011 m



0.0100 m

mionized



moriginal

mionized



moriginal

OSMOTIC PRESSURE

Osmosisis the spontaneous process by which the solvent molecules pass through a semi-

permeable membrane from a solution of lower concentration of solute into a solution of

higher concentration of solute. A semipermeable membrane(e.g., cellophane) separates

14-15

(Enrichment, continued)

To simplify the notation, we denote
the weak acid as HA and its anion as
A. The reaction summary used here
to analyze the extent of reaction was
introduced in Chapter 11.

Osmosis is one of the main ways in
which water molecules move into and
out of living cells. The membranes and
cell walls in living organisms allow
solvent to pass through. Some of these
also selectively permit passage of ions
and other small solute particles.