1. Unit 5- lecture 5
Using the pH scale to characterize acids and bases

2. The pH scale is a way of expressing the strength of acids and bases
The pH scale is a way of expressing the strength of acids and bases. It is based on the concentration of H+ each pH unit is a factor of 10 for [H+] < 7 = acid 7 = neutral > 7 = base

3. Equation to relate pH with H+
pH= - log[H+]
[H+] =10-pH
[H+] = H+ concentration of a substance in Molarity (M)

4. pH calculations – Solving for H+
Ex. 1: If the pH of Coke is 3, [H+] =?
[H+] = 10-pH
[H+] = 10-3
= .001 M = 1 x 10-3 M
Ex. 2: If the pH of ammonia is 11.5, [H+] =?
[H+] = 10-pH
[H+] = = 3.2 x M

5. pH calculations – Solving for pH
Ex 3: If [H+] = 2.5 X M, what is the pH?
pH = - log [H+]
pH = -log [2.5 X ]
pH = 9.6
Example 4: If [H+] = 8.0 X 10-5 M, what is the pH?
pH = - log [8.0 X 10-5 ]
pH = 4.1

6. Because pH is on a logarithmic scale… this means that for every 1 point change in pH the [H+] changes by a power of 10
Example: when pH change from 1 to 3…
- it is a 100 times change in H+
- (less acidic by 100 times)
When pH changes from 12 to 9
- it is a 1000 times change in H+
- more acidic by 1000 times.

7. 1. If a substance has a pH of 2, how many more times acidic is it then a substance with a pH of 6?
2. If a substance has a pH of 4, how many more times Hydrogen concentration does is it have
than a substance with a pH of 7?
3. Which substance is more acidic, that with a pH of 2 or a pH of 5. How many more times acidic is it?

8. pOH
Since acids and bases are opposites, pH and pOH are opposites!
So if something is acidic pOH is > 7, if its basic pOH is < 7, and if its neutral pOH = 7.
pOH looks at the perspective of a base
pOH = - log [OH-] [OH-] = 10-pOH
Since pH and pOH are on opposite ends,
pH + pOH = 14

9. pH
[H+]
[OH-]
pOH

10. Ex. 1: What is the pH of a solution with a pOH of 12.5?
pH + pOH = 14
pH = 1.5, acid
Ex 2: What is the pOH of a solution with an [OH-] of 5 x10-3 ?
pOH = -log [OH-]
= -log [5x10-3] = 2.3, base

11. Ex 3: What is the [OH-] of a solution with a pOH of 11.5?
[OH-] = 10-pOH =
= 3.16 x M , acid
Ex 4: What is the [OH-] of a solution with a pOH of 4.5? =
= M or 3.2 x10-5 M , base

12. All water solutions contain both H3O+ and OH-
In a neutral solution
[H+] = [OH-]
In an acidic solution
[H+] > [OH-]
In a basic solution
[H+] < [OH-]