1. Calculating pH from the Water Constant

2. Ionization of Water Constant
Remember: water will naturally break down into equal ions in water:
2H2O  H3O+ (aq) OH- (aq)
This is called the ionization of water.

3. Ionization of Water Constant
There is a set value, a constant, for this called the Kw.
Note: we represent the concentration ratio using brackets [ ]
Kw = [H3O+] [OH-]
( 1.00 X 10-7)(1.00 X 10-7) = 1.00 X 1014

4. We can use simple bar graphs to represent ion concentration.
H3O+ OH-
pH 6 pH 7 pH 8

5. Pure Water is Neutral (pH = 7)

6. Are the following solutions ACIDIC, BASIC, or NEUTRAL?
H+ = 1.0 x 10-6 M , OH- = 1.0 x 10-8 M
2. H+ = 1.0 x M , OH- = 1.0 x 10-4 M
3. H+ = 1.0 x 10-4 M , OH- = 1.0 x M

7. Are the following solutions ACIDIC, BASIC, or NEUTRAL?
4. H+ = 1.0 x 10-2 M
5. H+ = 4.5 x 10-9 M
6. OH- = 2.9 x M

8. Using Kw to Calculate Concentrations
You can use the Kw to calculate the concentration of either Hydronium ions or Hydroxide ions.
Example: a cola is slightly acidic. If the [H3O+] in a solution is 1.0 X 10-5 M, what is the [OH-] concentration of the solution and is it acidic, basic or neutral?

9. On Your Own: [H+] = 6.0 X 10-10 M [OH-] = 3.0 X 10-2 M
[OH-] = 3.0 X 10-2 M
[H+] = 2.0 X 10-7 M
[OH-] = 1.0 X 10-7 M