The Foundations of Chemistry

Thus far we have focused our attention on acids. Very few common weak bases are

soluble in water. Aqueous ammonia is the most frequently encountered example. From

our earlier discussion of bonding in covalent compounds (Section 8-8), we recall that there

is one unshared pair of electrons on the nitrogen atom in NH 3. When ammonia dissolves

in water, it accepts H
from a water molecule in a reversible reaction (Section 10-4). We

say that NH 3 ionizes slightly when it undergoes this reaction. Aqueous solutions of NH 3

are basic because OHions are produced.

Amines are derivatives of NH 3 in which one or more H atoms have been replaced by

organic groups, as the following structures indicate.

Thousands of amines are known, and many are very important in biochemical processes.

Low-molecular-weight amines are soluble weak bases. The ionization of trimethylamine,

for example, forms trimethylammonium ions and OHions.

Now let us consider the behavior of ammonia in aqueous solutions. The reaction of

ammonia with water and its ionization constant expression are

NH 3 
H 2 O 34 NH 4 

OH

and

Kb1.8 10 ^5

The fact that Kbfor aqueous NH 3 has the same value as Kafor CH 3 COOH is pure coin-

cidence. It does tell us that in aqueous solutions of the same concentration, CH 3 COOH

and NH 3 are ionized to the same extent. Table 18-6 lists Kband pKbvalues for a few

common weak bases. Appendix G includes a longer list of Kbvalues.

We use Kb’s for weak bases in the same way we used Ka’s for weak acids and pKbvalues

for weak bases in the same way we used pKavalues for weak acids.

EXAMPLE 18-14 pH of a Weak Base Solution

Calculate the [OH], pH, and percent ionization for a 0.20 Maqueous NH 3 solution.

Plan

Write the equation for the ionization of aqueous NH 3 and represent the equilibrium concen-

trations algebraically. Then, substitute into the Kbexpression and solve for [OH] and

[NH 3 ]ionized.

[NH 4 
][OH]



[NH 3 ]

H OH

H

 
OH

H
transfer

CH 3 CH 3

CH 3

N

trimethylamine

(CH 3 ) 3 N

CH 3 CH 3

CH 3

N

trimethylammonium ion

(CH 3 ) 3 NH

HH

H

N

ammonia

NH 3

H 3 CH

H

N

methylamine

CH 3 NH 2

H 3 CH

CH 3

N

dimethylamine

(CH 3 ) 2 NH

H 3 CCH 3

CH 3

N

trimethylamine

(CH 3 ) 3 N

NH 3 
 H 2 ONH 4 
 
 OH

The subscript “b” indicates that the
substance ionizes as a base. We do not
include [H 2 O] in the Kbexpression for
the same reasons described for Ka.

770 CHAPTER 18: Ionic Equilibria I: Acids and Bases

Trimethylamine, (CH 3 ) 3 N

Ammonia, NH 3