Copyright © Glencoe/McGraw-Hill, a division of The McGraw-Hill Companies, Inc.
Solving Problems: A Chemistry Handbook
Chemistry: Matter and Change
189
SOLVING PROBLEMS:
A CHEMISTRY HANDBOOK
CHAPTER 18
Practice Problems
9. Calculate the pH of the following strong acid or strong base
solutions.
a. 0.015M HCl
c. 2.5 ϫ 10 Ϫ4 M HNO 3
b. 0.65M KOH
d. 4.0 ϫ 10 Ϫ3 M Ca(OH) 2
Using pH to calculate K a If you know the pH and the concentration of a solution of a weak acid, you can calculate K a for the acid.
The following example problem illustrates this type of calculation.
Example Problem 18-5
Calculating K a from pH
The pH of a 0.200M solution of acetic acid (CH 3 COOH) is 2.72.
What is K a for acetic acid?
Write the equation for the ionization reaction. For simplicity, water
is omitted from the equation.
CH 3 COOH(aq) 3 H ϩ (aq) ϩ CH 3 COO Ϫ (aq)
The acid ionization constant expression is as follows.
K a ϭ
Use the pH to calculate [H ϩ ].
pH ϭ Ϫlog [H ϩ ]
[H ϩ ] ϭ antilog (ϪpH)
[H ϩ ] ϭ antilog (Ϫ2.72)
Use a log table or calculator to find that the antilog of Ϫ2.72 is
1.9 ϫ 10 Ϫ3 .
[H ϩ ] ϭ 1.9 ϫ 10 Ϫ3 M
In the ionization reaction, equal numbers of H ϩ ions and CH 3 COO Ϫ
ions are formed.
[CH 3 COO Ϫ ] ϭ [H ϩ ] ϭ 1.9 ϫ 10 Ϫ3 M
At equilibrium, [CH 3 COOH] equals the initial concentration of the
acid minus the moles per liter that dissociated.
[CH 3 COOH] ϭ 0.200M Ϫ [H ϩ ]
[H ϩ ][CH 3 COO Ϫ ]
ᎏᎏ
[CH 3 COOH]
▲