1. pH Scale & Indicators

2. pH Scale pH Scale: Expresses the strength of acids and bases
One pH unit = an inc/dec in [H3O+] by a ten fold
Neutral = 7
Acid < 7
Base > 7

3. pH = -log[H3O+] pOH = -log[OH-] pH + pOH = 14
Calculating pH
pH = -log[H3O+]
pOH = -log[OH-]
pH + pOH = 14

4. (Remember that the [ ] mean Molarity)
Calculating the pH
pH = - log [H+]
(Remember that the [ ] mean Molarity)
Example: If [H+] = 1 X pH = - log 1 X 10-10
pH = - (- 10)
pH = 10
Example: If [H+] = 1.8 X 10-5 pH = - log 1.8 X 10-5
pH = - (- 4.74)
pH = 4.74

5. Calculating pH What is the pH of 0.050 M HNO3? pH = -log[H3O+]
Acidic or basic?
Acidic

6. Calculating pH
What is the molarity of HBr in a solution that has a pOH of 9.6?
pH + pOH = 14
pH = 14
pH = 4.4
pH = -log[H3O+]
4.4 = -log[H3O+]
-4.4 = log[H3O+]
[H3O+] = 4.0  10-5 M HBr
Acidic

7. pH Indicators
Indicator: a compound that changes color in the presence of an acid or base
Most indicators are weak organic acids or bases that react with ions in solution.

9. pH Indicators Two Common Indicators are Litmus Phenolphthalein
Acids turn blue litmus red
Bases turn red litmus blue
Phenolphthalein
Colorless in acid
Faint pink in neutral
Bright pink in base
pH Meter: a device used to measure the exact pH value of a solution

10. pH Scale – Color Chart

11. pH 4.0
pH 5.0
pH 7.0
pH 6.0
pH 8.0
pH 9.0
pH 10.0