1. Chemistry 1011 TOPIC TEXT REFERENCE Acids and Bases
Masterton and Hurley Chapter 4.2 (Review), 13, 14.1, 15.1 (page 427), 21.2 (page589)
Chemistry 1011 Slot 5

2. 13.2 Water Ionization Constant
YOU ARE EXPECTED TO BE ABLE TO:
Identify KW as the ionization constant for water.
Define a solution as acidic, basic or neutral based upon the concentration of hydrogen ions.
Given KW and one of [H+] and [OH-], calculate the other.
Chemistry 1011 Slot 5

3. The Ionization of Water
Water molecules can be amphiprotic
H2O(l) + H2O(l) H3O+(aq) + OH-(aq)
This can be written more simply as
H2O(l) H+(aq) + OH-(aq)
The equilibrium constant for this reaction is
KW = [H+]x[OH-]
[H2O]
But [H2O] is constant, so KW = [H+]x[OH-]
Chemistry 1011 Slot 5

4. The Value of KW KW is known as the ionization constant for water
The value of KW is very small
KW = 1.0 x 10-14
In a pure water, [H+] = [OH-] = 1.0 x 10-7
This is a neutral solution
Chemistry 1011 Slot 5

5. Acidic and Basic Solutions
In an acidic solution,
[H+] > 1.0 x 10-7
[OH-] < 1.0 x 10-7
The product, KW is always = 1.0 x 10-14
In a basic solution,
[H+] < 1.0 x 10-7
[OH-] > 1.0 x 10-7
Chemistry 1011 Slot 5

6. Example
If a water sample has a hydrogen ion concentration of 4.0 x 10-5, what is the concentration of OH-? Is the solution acidic or basic?
Remember KW = [H+]x[OH-] = 1.0 x 10-14
If [H+] = 4.0 x 10 -5, then
[OH-] = 1.0 x = 2.5 x 10-10
4.0 x 10 -5
The sample is acidic
Chemistry 1011 Slot 5