The Foundations of Chemistry

Weak Bases Plus Salts of Weak Bases

Let us consider the second common kind of buffer solution, containing a weak base and

its salt. A solution that contains aqueous NH 3 and ammonium chloride, NH 4 Cl, a soluble

ionic salt of NH 3 , is typical. The NH 4 Cl is completely dissociated, but aqueous NH 3 is

only slightly ionized.

Both NH 4 Cl and aqueous NH 3 are sources of NH 4 ions. The completely dissociated

NH 4 Cl provides a high [NH 4 ]. This shifts the ionization equilibrium of aqueous NH 3

far to the left, as NH 4 ions combine with OH
ions to form nonionized NH 3 and H 2 O.

The result is that [OH
] is decreased significantly.

Solutions that contain a weak base plus a salt of the weak base are always less basic

than solutions that contain the same concentration of the weak base alone.

EXAMPLE 19-3 Weak Base/Salt of Weak Base Buffer Solution

Calculate the concentration of OH
and the pH of a solution that is 0.20 Min aqueous NH 3

and0.10 Min NH 4 Cl.

Plan

Write the appropriate equations for bothNH 4 Cl and NH 3 and the ionization constant expres-

sion for NH 3. Then, represent the equilibriumconcentrations algebraically, and substitute into

the Kbexpression.

Solution

The appropriate equations and algebraic representations of concentrations are

Substitution into the Kbexpression for aqueous NH 3 gives

Kb1.8 10 
^5 

The small value of Kbsuggests that xis very small. This leads to two assumptions.

(0.10x)(x)



(0.20
x)

[NH 4 ][OH
]



[NH 3 ]

rxn 1 NH 4 Cl NH 4 

0.10 M 0.10 M 0.10 M

(0.20 
 x) M x M x M

rxn 2 NH 3 H 2 ONH 4 

Cl

OH



Total [NH 4 ]  (0.10  x) M

rxn 1

rxn 2

NH 4 Cl NH 4 

NH 4  OH
 (reversible)

(to completion)







Cl

NH 3 H 2 O

H 2 O

798 CHAPTER 19: Ionic Equilibria II: Buffers and Titration Curves

Assumption Implication

x0.10, so (0.10x)
0.10 Most of the NH 4 comes from NH 4 Cl (rxn 1) and

very little NH 4 comes from ionization of NH 3 (rxn 2)

x0.20, so (0.20
x)
0.20 Very little of the NH 3 ionizes (rxn 2)