The Foundations of Chemistry

Solution

The equation for the ionization of aqueous ammonia and the algebraic representations of equi-
librium concentrations follow. Let x[NH 3 ]ionized.

NH 3 
H 2 O 34 NH 4 

OH

initial 0.20 M 0 M 
 0 M

change due to rxn x M 
x M 
x M

at equil (0.20x) Mx Mx M

Substitution into the ionization constant expression gives

Kb1.8 10 ^5 

Again, we can simplify this equation. The small value of Kbtells us that the base is only slightly
ionized, so we can assume that x0.20, or (0.20x)
0.20, and we have



0

x

.2

2

0

1.8 10 ^5 x^2 3.6 10 ^6 x1.9 10 ^3 M

Then [OH]x 1.9 10 ^3 M, pOH2.72, and pH 11.28.

[NH 3 ]ionizedx, so the percent ionization may be calculated.

% ionization100%100% 0.95% ionized

You should now work Exercises 54 and 110.

EXAMPLE 18-15 Household Ammonia

The pH of a household ammonia solution is 11.50. What is its molarity?

Plan

We are given the pH of an aqueous NH 3 solution. Use pH
pOH14.00 to find pOH,
which we can convert to [OH]. Then, complete the ionization reaction summary, and substi-
tute the representations of equilibrium concentrations into the Kbexpression.

Solution

At equilibrium pH11.50; we know that pOH2.50, so [OH] 10 2.503.2 10 ^3 M.
This [OH] results from the reaction, so we can write the change line. Then, letting xrepre-
sent the initialconcentration of NH 3 , we can complete the reaction summary.

1.9 10 ^3



0.20

[NH 3 ]ionized



[NH 3 ]initial

(x)(x)



(0.20x)

[NH 4 
][OH]



[NH 3 ]

The value of xis only about 1% of

the original concentration, so the

assumption is justified.

18-4 Ionization Constants for Weak Monoprotic Acids and Bases 771

TABLE 18-6 Ionization Constants and pKbValues for Some Weak Bases

Base Ionization Reaction Kbat 25°C pKb

ammonia NH 3
H 2 O 34 NH 4

OH 1.8 10 ^5 4.74

methylamine (CH 3 )NH 2
H 2 O 34 (CH 3 )NH 3

OH 5.0 10 ^4 3.30
dimethylamine (CH 3 ) 2 NH
H 2 O 34 (CH 3 ) 2 NH 2

OH 7.4 10 ^4 3.13

trimethylamine (CH 3 ) 3 N
H 2 O 34 (CH 3 ) 3 NH

OH 7.4 10 ^5 4.13
pyridine C 5 H 5 N
H 2 O 34 C 5 H 5 NH

OH 1.5 10 ^9 8.82

aniline C 6 H 5 NH 2
H 2 O 34 C 6 H 5 NH 3

OH 4.2 10 ^10 9.38

Measurement of the pH of a

solution of household ammonia.

C NH 2

C

CC

CC

H

HH

H

H

aniline

C

CC

CC

H

HH

N H

H

pyridine