1. The pH Scale The Power or Potential of Hydrogen
Topic 8.3

2. The Nature of Science Occam’s razor
a scientific and philosophic rule requiring that the simplest of competing theories be preferred to the more complex
the pH scale was proposed to simplify the comparison of concentrations of H+ ions in solution
based on the log10 scale
the base 10 logarithm scale is simply an exponential scale (scientific notation) written in a different form

3. pH scale
the pH scale is a way of expressing the strength of acids and bases
instead of using very small numbers, we just use the NEGATIVE power of 10 on the Molarity of the H+ ion
under 7 = acid = neutral over = base

4. pH of 1 is 100x more acidic than pH of 3
[H+] ions in scientific notation
[H+] ions traditionally
1x100
1.0 1
1x10-1
0.1 2
1x10-2
0.01 3
1x10-3
0.001 4
1x10-4
0.0001 5
1x10-5 6
1x10-6 7
1x10-7 8
1x10-8 9
1x10-9 10
1x10-10 11
1x10-11 12
1x10-12 13
1x10-13 14
1x10-14
pH of 1 is 100x more acidic than pH of 3
pH of 4 is 1,000x more basic than pH of 1
pH of 6 is 10,000 less concentrated [H+] than pH of 2

5. Properties of Water

6. water will self-ionize to a very small extent
in pure water at 25°C, the [concentration] of H+ (or H3O+) and OH- ions are equal
[H3O+] = [OH-]
both have a concentration of 1.0 x 10-7 M
the ionic product constant (Kw) for water at 298 K is:
Kw = [H+] [OH-]
= 1.0 x 10-7 x 1.0 x 10-7
= 1.0 x 10-14

7. acid solutions have a greater concentration of H+ ions (or H3O+) than OH- ions
basic solutions are opposite and have more OH-, referred to as alkaline solutions when in water

8. pH calculations – Solving for pH
pH = - log [H+]
[H+] = 10-pH
Example: If [H+] = 1 X 10-10, what is the pH?
pH = - log 1 X 10-10
pH = - (- 10)
pH = 10
Example: If [H+] = 1.8 X 10-5
pH = - log 1.8 X 10-5
pH = - (- 4.74)
pH = 4.74

9. Try These! Find the pH of these:
A 0.15 M solution of hydrochloric (HCl) acid
pH = .82
2) A 3.00 X 10-7 M solution of nitric acid (H2NO3)
pH = 6.5

10. pH calculations – Solving for H+
If the pH of Coke is 3.12, what is the [H+]?
10-pH = [H+]
[H+] =
= 7.6 x 10-4 M

11. A solution has a pH of 8.5. What is the Molarity of hydrogen ions in the solution?
pH = - log [H+]
= [H+]
3.16 X 10-9 = [H+]

12. solving for pOH
since acids and bases are opposites, pH and pOH are opposites
pOH does not really exist, but it is useful for changing bases to pH
pOH looks at the perspective of a base
pOH = - log [OH-]
Since pH and pOH are on opposite ends,
pH + pOH = 14

13. What is the pH of the 0.0010 M NaOH solution?
[OH-] = M
pOH = - log
pOH = 3
pH = 14 – 3 = 11

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

16. pH testing There are several ways to test pH
Blue litmus paper (turns red = acid)
Red litmus paper (turns blue = basic)
pH paper (multi-colored)
pH meter (7 is neutral, <7 acid, >7 base)
Universal indicator (multi-colored)
Indicators like phenolphthalein
Natural indicators like red cabbage, radishes

17. pH meter tests the voltage of the electrolyte
converts the voltage to pH
must be calibrated with a buffer solution and stored in a solution

18. pH indicators
indicators are dyes that can be added that will change color in the presence of an acid or base.
some indicators only work in a specific range of pH
once the drops are added, the sample is ruined
some dyes are natural, like radish skin or red cabbage