1. Do Now: Identify the acids and bases in the following equations:
NH4+(aq) + H2O(l) ↔ NH3(aq) + H3O+(aq)
NH3(aq) + H3O+(aq) ↔ NH4(aq) + H2O(l)
CH3COO-(aq) + H2O(l) ↔ CH3COOH(aq) + OH-(aq)
CH3COOH(aq) + OH-(aq) ↔ CH3COO-(aq) + H2O(l)

2. Kw, [OH-] & [H3O+]
201 Chemistry

3. Kaupapa / Learning Outcomes
Understand what Kw is and use it in calculations.
Know how to calculate pH of acids & bases

6. Dissociation constant of water, Kw
Pure water is in equilibrium
2H2O(l) H3O+(aq) OH(aq)
The equilibrium constant is: [H3O+] [OH-] = 1 x 10-14
Water is not included because you can’t have a concentration of a pure substance. Similarly solids are not used in the calculation of equilibrium constants

7. Kw
If [H3O+] [OH-] = 1 x 10-14
then
In pure water
[H3O+] = [OH-] = 1 x 10-7

8. Kw, [H3O+] & [OH-] Adding an acid to water increases [H3O+]
This decreases [OH-] (Le Chetaliers Principle) so that the product of the two concentrations (Kw) remains unchanged.
In other words given an H3O+ or OH- concentration we can use the
Kw = [H3O+] [OH-] expression to find an unknown concentration
Kw = 1x 10-14

9. Calculating [H3O+] & [OH-]
Example:
[H3O+] = 2.5 x 10-3 mol L-1 find the [OH-]
Rearrange Kw = [H3O+] x [OH] to find the OH- concentration
[OH] =
= 4.0 x mol L-1

10. Complete the following table using the previous Kw formula.
[H3O+]
[OH]
acid or base?
2.4 x 10-3
0.0175
0.0036
5.4 x 10-10
4.167 x 10-12
acid
5.71 x 10-13
acid
base
2.78 x 10-12
1.85 x 10-5
acid

11. pH

12. Calculating pH
pH = -log10[H3O+]

13. Have a Go pH = -log10[H3O+] Calculate the pH if [H3O+] =
mol L-1
0.05 mol L-1
0.01 mol L-1
0.371 mol L-1
pH = -log10[H3O+]

14. Answers pH = -log10[H3O+] Calculate the pH if [H3O+] = 0.0040 mol L-1
2.40
1.30
2.00
0.431
pH = -log10[H3O+]

15. Calculating [H3O+] from pH
[H3O+] = 10-pH

16. Have a go [H3O+] = 10-pH Calculate the [H3O+] if the pH = 1.08 2.97
12.5
4.67
[H3O+] = 10-pH

17. Answers [H3O+] = 10-pH Calculate the [H3O+] if the pH = 1.08 2.97 12.5
4.67
molL-1
1.07 x 10-3 molL-1
3.16 x molL-1
2.14 x 10-5 molL-1
[H3O+] = 10-pH

18. Calculating the pH of bases
Use the [OH-] to calculate [H3O+]
[OH-] x [H3O+] = 10-14
[H3O+] = / [OH-]
Then calculate pH

19. Have a Go Calculate the pH if [OH-] = 0.0040 mol L-1 0.05 mol L-1

20. Answers Calculate the pH if [OH-] = 0.0040 mol L-1 0.05 mol L-1
11.6
12.7 13
13.6