5 Steps to a 5 AP Chemistry

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 Kbproblem, and OH-will be produced. The equilibrium is:

Determining Kbfrom Ka(using Kw=KaKb=1.0 × 10 −^14 gives:

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 con-

tains 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 signifi-

cant 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 presence

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 solu-

tion, but also interfere with each other. Predictions require a comparison of the Kvalues for

[

.

xx

x

K

][ ]

0.400− b

==×56 10−^10

CHO 232 HO 2 OH HCHO 232

0 400

−−++

−++



. xxx

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