The Foundations of Chemistry

Let us consider the reaction that occurs when a weak acid, such as acetic acid, is dissolved

in water. The equation for the ionization of acetic acid is

CH 3 COOH(aq)
H 2 O(
) 34 H 3 O
(aq)
CH 3 COO(aq)

The equilibrium constant for this reaction could be represented as

Kc

We should recall that the thermodynamic definition of Kis in terms of activities. In dilute

solutions, the activity of the (nearly) pure H 2 O is essentially 1. The activity of each

dissolved species is numerically equal to its molar concentration. Thus the ionization

constantof a weak acid, Ka, does not include a term for the concentration of water.

We often use HA as a general representation for a monoprotic acid and Afor its

conjugate base. The equation for the ionization of a weak acid can be written as

HA 34 H

A

For example, for acetic acid, we can write either

Ka

or

Ka1.8 10 ^5

This expression tells us that in dilute aqueous solutions of acetic acid, the concentration

of H 3 O
multiplied by the concentration of CH 3 COOand then divided by the concen-

tration of nonionizedacetic acid is equal to 1.8 10 ^5.

Ionization constants for weak acids (and bases) must be calculated from experimentally

determined data. Measurements of pH, conductivity, or depression of freezing point provide

data from which these constants can be calculated.

EXAMPLE 18-7 Calculation of Kaand pKafrom Equilibrium

Concentrations

Nicotinic acid is a weak monoprotic organic acid that we can represent as HA.

HA
H 2 O 34 H 3 O

A

[H 3 O
][CH 3 COO]



[CH 3 COOH]

[H
][A]



[HA]

[H 3 O
][CH 3 COO]



[CH 3 COOH][H 2 O]

Recall that pKameans log Ka.

762 CHAPTER 18: Ionic Equilibria I: Acids and Bases

TABLE 18-4 Ionization Constants and pKaValues for Some Weak Monoprotic

Acids

Acid Ionization Reaction Kaat 25°C pKa

hydrofluoric acid HF
H 2 O 34 H 3 O

F 7.2 10 ^4 3.14

nitrous acid HNO 2 
H 2 O 34 H 3 O

NO 2  4.5 10 ^4 3.35

acetic acid CH 3 COOH
H 2 O 34 H 3 O

CH 3 COO 1.8 10 ^5 4.74

hypochlorous acid HOCl
H 2 O 34 H 3 O

OCl 3.5 10 ^8 7.45

hydrocyanic acid HCN
H 2 O 34 H 3 O

CN 4.0 10 ^10 9.40

The thermodynamic approach is that
the activity of the (nearly) pure H 2 O is
essentially 1. The activity of each
dissolved species is numerically equal
to its molar concentration.