1. Chapter 10 Acids and Bases
The pH Scale

2. pH Scale The pH scale: Is used to indicate the acidity of a solution.
Has values that usually range from 0 to 14.
Indicates an acidic solution when the values are less than 7.
Indicates a neutral solution with a pH of 7.
Indicates a basic solution when the values are greater than 7.

3. pH Range Acidic Neutral Basic
Neutral
[H3O+]>[OH-] [H3O+] = [OH-] [H3O+]<[OH-]
Acidic
Basic

5. pH Scale
pH is:
Defined as the negative log of the hydrogen ion concentration.
pH = - log [H3O+]
The value of the negative exponent of [H3O+] for concentrations with a coefficient of 1.
[H3O+] = 1 x pH = 4.0
[H3O+] = 1 x pH = 11.0

6. Calculations of pH A calculator can be used to find the pH of a
solution with a [H3O+] of 1 x 10-3 as follows.
1. Enter 1 x by pressing 1 (EE) 3 (+/-).
The EE key gives an exponent of 10 and the +/- key changes the sign.
2. Press the log key to obtain log 1 x 10-3.
log (1 x 10-3) = -3
3. Press the +/- key to multiply by –1.
(-3) +/- = 3

7. Significant Figures in pH
When expressing log values, the number of decimal places in the pH is equal to the number of significant figures in the coefficient of [H3O+].
[H3O+] = 1 x 10-4 pH = 4.0
[H3O+] = 1.0 x 10-6 pH = 6.00
[H3O+] = 2.4 x 10-8 pH = 7.62

8. Calculating [H3O+] from pH
The [H3O+] can be expressed with the pH as the negative power of 10.
[H3O+] = 1 x 10-pH
If the pH is 3.0, the [H3O+] = 1 x 10-3
On a calculator
1. Enter the pH value
2. Use (+/-) to change sign 3.0 (+/-) = -3.0
3. Use the inverse log key (or 10x) to obtain
the [H30+]. = 1 x 10-3 M

9. pOH The pOH of a solution: Is related to the [OH-]. Is similar to pH.
pOH = - log [OH-]
Added to the pH value is equal to 14.0.
pH + pOH = 14.0

10. pH and pOH
The sum of the pH and pOH values is equal to 14.0 for any aqueous solution.