pH and pOH
Jan 05, 2016
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