The Foundations of Chemistry

Because ionization constants are equilibrium constants for ionization reactions, their
values indicate the extents to which weak electrolytes ionize. At the same concentrations,
acids with larger ionization constants ionize to greater extents (and are stronger acids)
than acids with smaller ionization constants. From Table 18-4, we see that the order of
decreasing acid strength for these five weak acids is

HFHNO 2 CH 3 COOHHOClHCN

Conversely, in Brønsted–Lowry terminology (Section 10-4), the order of increasing base
strength of the anions of these acids is

FNO 2 CH 3 COOOClCN

If we know the value of the ionization constant for a weak acid, we can calculate the
concentrations of the species present in solutions of known initial concentrations.

EXAMPLE 18-10 Calculation of Concentrations from Ka

Calculate the concentrations of the various species in 0.10 Mhypochlorous acid, HOCl. For
HOCl, Ka3.5 10 ^8.

Plan

We write the equation for the ionization of the weak acid and its Kaexpression. Then we repre-
sent the equilibriumconcentrations algebraically and substitute into the Kaexpression.

Solution

The equation for the ionization of HOCl and its Kaexpression are

HOCl
H 2 O 34 H 3 O

OCl and Ka3.5 10 ^8

We would like to know the concentrations of H 3 O
, OCl, and nonionized HOCl in solu-
tion. An algebraic representation of concentrations is required, because there is no other obvious
way to obtain the concentrations.
Let xmol/L of HOCl that ionizes. Then, write the “change” line and complete the reac-
tion summary.

HOCl 
H 2 O 34 H 3 O

OCl

initial 0.10 M 
 0 M 0 M

change due to rxn x M 
x M 
x M

at equil (0.10x) Mx Mx M

[H 3 O
][OCl]



[HOCl]

Recall that in Brønsted–Lowry

terminology, an acid forms its

conjugate base by losing H
.

We have written the formula for

hypochlorous acid as HOCl rather

than HClO to emphasize that its

structure is HXOXCl.

We neglect the 1.0 10 ^7 mol/L of

H 3 O
produced by the ionization of

purewater. Recall (see Section 18-2)

that the addition of an acid to water

suppresses the ionization of H 2 O, so

[H 3 O
] from H 2 O is even less than

1.0 10 ^7 M.

18-4 Ionization Constants for Weak Monoprotic Acids and Bases 765

HA H 2 O H 3 O
 A

initial

change due to rxn

at equil 0.103 M 0.012^ M

0.012 M

0.115 M


 0.012^ M

0.012^ M

0.012^ M

At equilibrium, [H 3 O
]  0.012 M so

≈ 0 M 0 M

1

6

(^35)
4 2
4.[HA]equil[HA]orig
[HA]chg0.115 M
(0.012 M)0.103 M
5.Formation of 0.012 MH 3 O
also gives 0.012 MA
6.[A]equil[A]orig
[A]chg 0 M
0.012 M0.012 M