Chap_18 - Chapter 18 Equilibria in Solutions of Weak Acids...

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Chapter 18
Equilibria in Solutions of
Weak Acids and Bases
The list of the strong acids and bases available in chemistry, as you now know, is very short. Most acids and
bases are weak, and there are thousands of them. Those that are somewhat soluble in water do not come even
close to being 100% ionized in solution. But they vary widely in how weak they are, so special equilibrium
constants for these substances—acid or base ionization constants—have been devised whose values tell us at a
glance how relatively strong or weak such acids and bases are.
Weak acids or bases and their conjugates have life-and-death roles in nature because they help to main-
tain the pH values of the fluids of living systems within very narrow limits. Our study of buffers explains how
this all works. If you are planning to enter any one of the health sciences, buffers might be the most important
single topic in this chapter, given their importance in human health and disease.
Learning how to do the various calculations in this chapter is of paramount importance. If you can do
the calculations, and doing them is not just mechanical, you almost certainly understand the concepts. The
large number of worked examples have been very carefully prepared. If you master one kind before going on
to the next, the way to success will be smooth. The trickiest parts involve making simplifying assumptions
about what we can safely ignore in certain calculations. It’s particularly important that you understand these
assumptions and can judge when to make them, because they really do simplify the calculations.
Good luck in your study of this vital chapter. There are very few places in either the chemical or the
biological sciences where the equilibria of weak acids and bases in water are not important.
Learning Objectives
Throughout your study of this chapter, keep in mind the following objectives.
1 To learn to write the ionization equilibrium for a weak acid and use the equation to write
the appropriate expression for the acid ionization constant, K
a.
2 To learn to write the ionization equilibrium for a weak base and use the equation to write
the appropriate expression for the acid ionization constant, K
b.
3 To be able to calculate the value of p K
a
or p K
b
given K
a
or K
b
, respectively, for a weak
acid or base.
4 To learn how the values of K
a
and K
b
for a conjugate acid–base pair are related and how
one can be calculated given the other.
5 To learn how p K
a
and p K
b
are related for members of an acid-base conjugate pair.
6 For a solution containing a single solute, to learn how to calculate the value of K
a
or K
b
given the initial molarity and the measured pH.
7 To learn how to use the percentage ionization of an acid or base to calculate K
a
or K
b
, re-
spectively.

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8 For a solution containing a single solute, to learn how to calculate equilibrium concen-
trations, the solution’s pH, and the solute’s percentage ionization from the K
a
or K
b
value and the initial concentration of the solute.
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