Chapter 16 Text

16.9 Acid-Base Properties of Salt Solutions 641
(b)K& for NH3 is listed in Table 16.4 and in Appendix D as Kh = 1.8 X 1CT5. Using
Equation 16.40, we can calculate Ka for the conjugate acid, NH4+:
Kb 1.8 x 1(
PRACTICE EXERCISE
(a) Which of the following anions has the largest base-dissociation constant:
/ PO43 / or N3~? (b) The base quinoline has the following structure:
oio
Its conjugate acid is listed in handbooks as having a pKa of 4.90. What is the base-dissociation
constant for quinoline?
Answers: (a) PO43~(K& - 2.4 x 1(T2}; (b) 7.9 X 10~10
16.9 Acid-Base Properties of Salt Solutions
Even before you began this chapter, you were undoubtedly aware of many substances
that are acidic, such as HNO3, HC1, and H2SO4, and others that are basic,
such as NaOH and NHs. However, our recent discussions have indicated that
ions can also exhibit acidic or basic properties. For example, we calculated Ka
for NH4+ and K^ for F~ in Sample Exercise 16.16. Such behavior implies that salt
solutions can be acidic or basic. Before proceeding with further discussions of
acids and bases, let's examine the way dissolved salts can affect pH.
We can assume that when salts dissolve in water, they are completely dissociated;
nearly all salts are strong electrolytes. Consequently, the acid-base properties
of salt solutions are due to the behavior of their constituent cations and
anions. Many ions are able to react with water to generate H+(ag) or OHT(flq).
This type of reaction is often called hydrolysis. The pH of an aqueous salt solution
can be predicted qualitatively by considering the ions of which the salt is
composed. \
Bassam Z. Shakhashiri, "Hydrolysis:
Acidic and Basic Properties of Salts,"
Chemical Demonstrations: A
Handbook for Teachers of Chemistry,
Vol. 3 (The University of Wisconsin
Press, Madison, 1989) pp. 1 03-108.
An Anion's Ability to React with Water
In general, an anion, X~, in solution can be considered the conjugate base of an
acid. For example, Cl~ is the conjugate base of HC1 and C2H3O2^ is the conjugate
base of HC2H3O2. Whether an anion reacts with water to produce hydroxide
depends upon the strength of the acid to which it is conjugate. To identify the acid
and assess its strength, we can simply add a proton to the anion's formula:
X" plus a proton gives HX
H the acid determined in this way is one of the strong acids listed at the beginning
of Section 16.5, then the anion in question will have a negligible tendency
to abstract protons from water (Section 16.2) and the following equilibrium will
lie entirely to the left:
H2O(0 [16.42]
Consequently, the anion X will not affect the pH of the solution. The presence
°f Cr in an aqueous solution, for example, does not result in the production of
any OH" and does not affect the pH.
/