pH and pOH

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.

pH

pH is a measure of the concentration of hydronium ions in a solution.

pH = -log [H3O+] or

pH = -log [H+]

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)

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

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.

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.

The pH Scale

Acid Neutral Base

0 7 14

pOH

pOH is a measure of the concentration of hydroxide ions in a solution.

pOH = -log [OH-]

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

How are pH and pOH related?

At every pH, the following relationships hold true:

[H+] • [OH-] = 1 x 10-14 M

pH + pOH = 14

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

Example 2: The pH of a solution is measured to be 8.86. What is the [H+] in this solution?

pH = -log [H+] 8.86 = -log [H+] -8.86 = log [H+] [H+] = antilog (-8.86) [H+] = 10-8.86

[H+] = 1.4 x 10-9 M

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

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

Buffered Solutions

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

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-

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-

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

Acid Base Titration

A very accurate method to measure concentration.

Acid + Base Salt + Water

H+ + OH- H2O

Moles H+ = Moles OH-

Titration Formula

(Volume Acid)(Molar Conc. Acid)

=

(Volume Base)(Molar Conc. Base)

Buret

Solution with Indicator