1. Self Ionization of Water and the pH Scale

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

3. pH Scale 7 14 NEUTRAL ACIDIC BASIC
• scale used by chemists to measure the acidity of a substance; a measure of the amount of H+ in the solution.
• scale goes from 0 – 14
NEUTRAL 7
OH- = H3O+ 14
ACIDIC
BASIC
OH- < H3O+ (We are counting how many hydronium ions are in a solution!)
OH- > H3O+ (We are counting how many hydroxide ions are in a solution!)

4. pH Scale
pH describes a balance between hydroxide ions (OH-) and hydronium ions (H3O+)
Note: [H+] and [H3O+] are equivalent! We will use them interchangeably.
pH stands for “parts hydrogen ion”
pOH stands for “parts hydroxide ion”

5. Is It An Acid or a Base?
pH = 4
pH = 10
pH = 7

6. The pH scale is not linear!
The lower the pH, the higher the concentration of hydronium ions.
The pH scale changes 10 times between each step.

7. Example Problem:
How many times more acidic is solution A than solution B?
A B
pH = 4 pH = 6
pH = 3 pH = 5
pH = 8 pH = 11
pH = 7 pH = 4
The number of zeros = the difference in pH.
Ex =2 so, times.

8. Estimating pH from [H3O+]
The pH of a solution can be estimated by looking at the concentration of hydronium (H3O+).
The exponent is the estimated pH of the substance!
Example:
[H3O+] = 1.0 x 10-3 M
pH = 3

9. Estimating pH from [H3O+]
The pH of a solution can be estimated by looking at the concentration of hydronium (H3O+).
Example:
[H3O+] = 1.0 x M
pH = 12

10. Estimating pH from [H3O+]
The pH of a solution can be estimated by looking at the concentration of hydronium (H3O+).
Example:
[H3O+] = 3.1 x M

11. Estimating pH from [H3O+]
The pH of a solution can be estimated by looking at the concentration of hydronium (H3O+).
Example:
[H3O+] = 3.1 x M
pH = a little less than 12

12. Estimating [H3O+] from pH
Example:
pH = 9.0 So, [H3O+] = 1.0 x 10-9 M

13. Estimating [H3O+] from pH
You try:
pH = 7.0
pH = 2.0
pH = 8.0