pH Scale & Indicators

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

pH Scale

Calculating pH

pH = -log[H3O+]

pOH = -log[OH-]

pH + pOH = 14

Calculating the pHpH = - log [H+]

(Remember that the [ ] mean Molarity)

Example: If [H+] = 1 X 10-10

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

Calculating pH

□What is the pH of 0.050 M HNO3?

pH = -log[H3O+]

pH = -log[0.050]

pH = 1.3

Acidic or basic?Acidic

Calculating pH

□What is the molarity of HBr in a solution that has a pOH of 9.6?

pH + pOH = 14

pH + 9.6 = 14

pH = 4.4

Acidic

pH = -log[H3O+]

4.4 = -log[H3O+]

-4.4 = log[H3O+]

[H3O+] = 4.0 10-5 M HBr

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.

pH Indicators

□Two Common Indicators are□Litmus

□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

pH Scale – Color Chart

pH 4.0 pH 5.0 pH 6.0 pH 7.0

pH 8.0 pH 9.0 pH 10.0