The Foundations of Chemistry

It is reasonable to assume that x(from the ionization of NH 3 ) is small, because NH 3 is a weak
base (rxn 2), and its ionization is further suppressed by the high concentration of NH 4 formed
by the soluble salt, NH 4 Cl (rxn 1).
Introducing these assumptions gives



0

0

.

.

1

2

0

0

x

1.8^10 
^5 M and x3.6^10 
^5 M

xM [OH
]3.6 10 
^5 M so pOH4.44 and pH9.56

You should now work Exercise 10.

In Example 18-14 we calculated [OH
] and pH for 0.20 Maqueous NH 3. Compare those
results with the values obtained in Example 19-3 (Table 19-2). The concentration of OH

is 53 times greaterin the solution containing only 0.20 Maqueous NH 3 than in the solu-
tion to which 0.10 mol/L NH 4 Cl has been added. This is another demonstration of the
common ion effect.
We can derive a relationship for [OH
] in a solution containing a weak base, B, plus
a salt that contains the cation, BH, of the weak base, just as we did for weak acids. In
general terms the equation for the ionization of a monoprotic weak base and its Kbexpres-
sion are

BH 2 O 34 BHOH
 and Kb

Solving the Kbexpression for [OH
] gives

[OH
]Kb

Taking the logarithm of both sides of the equation gives

log [OH
]log Kblog

Multiplication by
1 and rearrangement gives another form of the Henderson–Hasselbalch
equationfor solutions containing a weak base plus a salt of the weak base.

pOHpKblog where pKb
log Kb (base/salt buffer)

[BH]



[B]

[B]



[BH]

[B]



[BH]

[BH][OH
]



[B]

19-1 The Common Ion Effect and Buffer Solutions 799

TABLE 19-2 Comparison of [OH
]and pH in Ammonia and Ammonium

Chloride—Ammonia Solutions

Solution % NH 3 Ionized [OH
]pH

0.20 Maq NH 3 0.95% 1.9 10
^3 M 11.28
0.20 Maq NH 3 pH
1.72
and 0.018% 3.6 10
^5 M 9.56

0.10 Maq NH 4 Cl





















The percent ionization of NH 3 in this

solution is

100%0.018%.

3.6 10 
^5 Mionized



0.20Moriginal

B and BHrepresent the weak base

and its conjugate acid, respectively—

for example, NH 3 and NH 4 
.

The two solutions in Table 19-2, in

the presence of universal indicator.

The NH 3 solution is on the left.

Can you calculate the percentage of

NH 3 that is ionized in these two

solutions?