1. pH and pOH

2. Ionization of water
Experiments have shown that pure water ionizes very slightly:
2H2O  H3O+ + OH-
Measurements show that: [H3O+] = [OH-]=1 x 10-7 M
Pure water contains equal concentrations of H3O+ + OH-, so it is neutral.

3. pH pH = -log [H3O+] pH = -log [H+]
pH is a measure of the concentration of hydronium ions in a solution.
pH = -log [H3O+] or
pH = -log [H+]

4. Sig. Figs. for Logarithms
The rule is that the number of decimal places in the log is equal to the number of significant figures in the original number.
Example:
[H+] = 1.0 x 10-9 M (2 significant figures)
pH = -log(1.0 x 10-9) = 9.00 (2 decimal places)

5. Example: What is the pH of a solution where [H3O+] = 1 x 10-7 M?
pH = -log [H3O+]
pH = -log(1 x 10-7)
pH = 7.0

6. Example: What is the pH of a solution where [H3O+] = 1 x 10-5 M?
pH = -log [H3O+]
pH = -log(1 x 10-5)
pH = 5.0
When acid is added to water, the [H3O+] increases, and the pH decreases.

7. Example: What is the pH of a solution where [H3O+] = 1 x 10-10 M?
pH = -log [H3O+]
pH = -log(1 x 10-10)
pH = 10.0
When base is added to water, the [H3O+] decreases, and the pH increases.

8. The pH Scale
Acid Neutral Base

9. pOH
pOH is a measure of the concentration of hydroxide ions in a solution.
pOH = -log [OH-]

10. Example: What is the pOH of a solution where [OH-] = 1 x 10-5 M?
pOH = -log [OH-]
pOH = -log(1 x 10-5)
pOH = 5.0

11. How are pH and pOH related?
At every pH, the following relationships hold true:
[H+] • [OH-] = 1 x M
pH + pOH = 14

12. Example 1: What is the pH of a solution where [H+] = 3.4 x 10-5 M?
pH = -log [H+]
pH = -log(3.4 x 10-5 M)
pH = 4.47

13. pH = -log [H+] 8.86 = -log [H+] -8.86 = log [H+]
Example 2: The pH of a solution is measured to be What is the [H+] in this solution?
pH = -log [H+]
8.86 = -log [H+]
-8.86 = log [H+]
[H+] = antilog (-8.86)
[H+] =
[H+] = 1.4 x 10-9 M

14. Example 3: What is the pH of a solution where [H+] = 5.4 x 10-6 M?
pH = -log [H+]
pH = -log(5.4 x 10-6)
pH = 5.27

15. Example 4: What is the [OH-] and pOH for the solution in example #3?
[H3O+][OH-]= 1 x 10-14
(5.4 x 10-6)[OH-] = 1 x 10-14
[OH-] = 1.9 x 10-9 M
pH + pOH = 14
pOH = 14 – 5.27 = 8.73

16. Buffered Solutions
A solution of a weak acid and a common ion is called a buffered solution.

17. Consider the following buffered solution…
HAc  H+ + Ac-
H2O  H+ + OH-
Add additional acid…(H+)
The H+ will combine with the Ac- producing HAc. There is an excess of Ac- from the common ion salt.
HAc  H+ + Ac-

18. Now, add additional base (OH-)
The OH- will combine with the H+ to produce water…
H2O  H+ + OH-
The H+ comes from the HAc
HAc  H+ + Ac-

19. Thus, the solution maintains it’s pH in spite of added acid or base.