5 Steps to a 5 AP Chemistry 2019

Equilibrium ❮ 223

The resultant solution, since strontium acetate is soluble, has 0.200 M Sr^2 + and 0.400 M
C 2 H 3 O 2 -.
Ions such as Sr^2 +, which come from strong acids or strong bases, may be ignored in this
type of problem. Ions such as C 2 H 3 O 2 - , from weak acids or bases, will undergo hydrolysis.
The acetate ion is the conjugate base of acetic acid (Ka = 1.8 × 10 -^5 ). Since acetate is not
a strong base this will be a Kb problem, and OH- will be produced. The equilibrium is:

++

−++

−−

xxx

CHOHOOHHCHO

0.400

2322 232

Determining Kb from Ka (using Kw = KaKb = 1.0 × 10 -^14 ) gives:

−

==× −

xx

x

K

[][]

0.400

b 5.6^1010

with x = 1.5 × 10 -^5 = [OH-], and pH = 9.180

Acidic/Basic Properties of Salts

The behavior of a salt will depend upon the acid–base properties of the ions present in the
salt. The ions may lead to solutions of the salt being acidic, basic, or neutral. The pH of a
solution depends on hydrolysis, a generic term for a variety of reactions with water. Some
ions will undergo hydrolysis, and this changes the pH.
The reaction of an acid and a base will produce a salt. The salt will contain the cation
from the base and the anion from the acid. In principle, the cation of the base is the conjugate
acid of the base, and the anion from the acid is the conjugate base of the acid. Thus, the salt
contains a conjugate acid and a conjugate base. This is always true in principle. In some cases,
one or the other of these ions is not a true conjugate base or a conjugate acid. Just because the
ion is not a true conjugate acid or base does not mean that we cannot use the ion as if it were.
The conjugate base of any strong acid is so weak that it will not undergo any sig-
nificant hydrolysis; the conjugate acid of any strong base is so weak that it, too, will not
undergo any significant hydrolysis. Ions that do not undergo any significant hydrolysis will
have no effect upon the pH of a solution and will leave the solution neutral. The pres-
ence of the following conjugate bases Cl-, Br-, I-, NO 3 - , ClO 3 - , and ClO 4 - will leave the
solution neutral. The cations from the strong bases, Li+, Na+, K+, Rb+, Cs+ Ca^2 +, Sr^2 +, and
Ba^2 +, while not true conjugate acids, will also leave the solution neutral. Salts containing a
combination of only these cations and anions are neutral.
The conjugate base from any weak acid is a strong base and will undergo hydrolysis in
aqueous solution to produce a basic solution. If the conjugate base (anion) of a weak acid
is in a salt with the conjugate of a strong base (cation), the solution will be basic, because
only the anion will undergo any significant hydrolysis. Salts of this type are basic salts. All
salts containing the cation of a strong base and the anion of a weak acid are basic salts.
The conjugate acid of a weak base is a strong acid, and it will undergo hydrolysis in an
aqueous solution to make the solution acidic. If the conjugate acid (cation) of a weak base is
in a salt with the conjugate base of a strong acid (anion), the solution will be acidic, because
only the cation will undergo any significant hydrolysis. Salts of this type are acidic salts. All
salts containing the cation of a weak base and the anion of a strong acid are acidic salts.
There is a fourth category, consisting of salts that contain the cation of a weak base with
the anion of a weak acid. Prediction of the acid–base character of these salts is less obvious,
because both ions undergo hydrolysis. The two equilibria not only alter the pH of the solution,