Page 262 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 17] ACID-BASE THEORY 251

the first two steps are done in the opposite order. See your owner’s manual. Do not use the minus key. (See the
Appendix.)

EXAMPLE 17.12. What is the difference in calculating the pH of a solution of a weak acid and that of a strong acid?
Ans. For a strong acid, the H 3 O concentration can be determined directly from the concentration of the acid. For a weak
+
+
acid, the H 3 O concentration must be determined first from an equilibrium constant calculation (Sec. 17.3); then the
pH is calculated.
−5
EXAMPLE 17.13. Calculate the pH of (a) 0.10 M HCl and (b) 0.10 M HC 2 H 3 O 2 (K a = 1.8 × 10 ).
Ans. (a) [H 3 O ] = 0.10 = 1.0 × 10 −1
+
pH = 1.00
(b) HC 2 H 3 O 2 + H 2 O −→ H 3 O + C 2 H 3 O 2 −
+
←−
Initial 0.10 0 0
Change −x x x
Equilibrium 0.10 − x x x
x 2 −5
= 1.8 × 10
0.10
x = 1.3 × 10 −3
pH = 2.89
From the way pH is defined and the value of K w , we can deduce that solutions with pH = 7 are neutral,
those with pH less than 7 are acidic, and those with pH greater than 7 are basic.
−
+
[H 3 O ] > [OH ] acidic pH < 7
[H 3 O ] = [OH ] neutral pH = 7
+
−
+
[H 3 O ] < [OH ] basic pH > 7
−
17.6. BUFFER SOLUTIONS
A buffer solution is a solution of a weak acid and its conjugate base or a weak base and its conjugate acid.
The main property of a buffer solution is its resistance to changes in its pH despite the addition of small quantities
of strong acid or strong base. The student must know the following three things about buffer solutions:
1. How they are prepared.
2. What they do.
3. How they do it.
A buffer solution may be prepared by the addition of a weak acid to a salt of that acid or addition of a weak
base to a salt of that base. For example, a solution of acetic acid and sodium acetate is a buffer solution. The
weak acid (HC 2 H 3 O 2 ) and its conjugate base (C 2 H 3 O 2 , from the sodium acetate) constitute a buffer solution.
−
There are other ways to make such a combination of weak acid plus conjugate base (Problem 17.26).
A buffer solution resists change in its acidity. For example, a certain solution of acetic acid and sodium
acetate has a pH of 4.0. When a small quantity of NaOH is added, the pH goes up to 4.2. If that quantity of NaOH
had been added to the same volume of an unbuffered solution of HCl at pH 4, the pH would have gone up to a
value as high as 12 or 13.
The buffer solution works on the basis of Le Chˆatelier’s principle. Consider the equation for the reaction of
acetic acid with water:

HC 2 H 3 O 2 + H 2 O −→ C 2 H 3 O 2 + H 3 O +
−
←−
excess huge excess limited
excess quantity
The solution of HC 2 H 3 O 2 and C 2 H 3 O 2 in H 2 O results in the relative quantities of each of the species in the
−
equation as shown under the equation. If H 3 O is added to the equilibrium system, the equilibrium shifts left to
+
use up some of the added H 3 O . If the acetate ion were not present to take up the added H 3 O , the pH would
+
+
drop. Since the acetate ion reacts with much of the added H 3 O , there is little increase in H 3 O and little drop
+
+