The Foundations of Chemistry

In the indicator method we prepare a series of solutions of known pH (standard solu-
tions). We add a universal indicator to each; solutions with different pH have different
colors (Figure 18-2). We then add the same universal indicator to the unknown solution
and compare its color to those of the standard solutions. Solutions with the same pH have
the same color.
Universal indicator papers can also be used to determine pH. A drop of solution is
placed on a piece of paper or a piece of the paper is dipped into a solution. The color of
the paper is then compared with a color chart on the container to establish the pH of the
solution.

IONIZATION CONSTANTS FOR WEAK MONOPROTIC

ACIDS AND BASES

We have discussed strong acids and strong bases. There are relatively few of these. Weak
acids are much more numerous than strong acids. For this reason you were asked to learn
the list of common strong acids (see Table 18-1). You may assume that nearly all other
acids you encounter in this text will be weak acids. Table 18-4 contains names, formulas,
ionization constants, and pKavalues for a few common weak acids; Appendix F contains
a longer list of Kavalues. Weakacids ionize only slightly in dilute aqueous solution. Our
classification of acids as strong or weak is based on the extent to which they ionize in dilute
aqueous solution.
Several weak acids are familiar to us. Vinegar is a 5% solution of acetic acid,
CH 3 COOH. Carbonated beverages are saturated solutions of carbon dioxide in water,
which produces carbonic acid.

CO 2 
H 2 O 34 H 2 CO 3

Citrus fruits contain citric acid, C 3 H 5 O(COOH) 3. Some ointments and powders used for
medicinal purposes contain boric acid, H 3 BO 3. These everyday uses of weak acids suggest
that there is a significant difference between strong and weak acids. The difference is that
strong acids ionize completely in dilute aqueous solution, whereas weak acids ionize only slightly.

18-4

Figure 18-2 Solutions containing

a universal indicator. A universal

indicator shows a wide range of

colors as pH varies. The pH values

are given by the black numbers.

These solutions range from quite

acidic (upper left) to quite basic

(lower right).

18-4 Ionization Constants for Weak Monoprotic Acids and Bases 761

1234

5678

9 10 11 12

How can you tell that carbonated

beverages are saturatedCO 2 solutions?

Would you think of using sulfuric or

nitric acid for any of these purposes?