1. pH Measure of acidity or alkalinity of substances in aqueous solutions
Note small p and big H
Follows on from Kc

2. Water reacts with itself H2O + H2O = H3O+ + OH-
Hydroxonium ion
OH-
Hydroxide ion

3. H2O + H2O = H3O+ + OH- if we look at Kc = [H3O+][OH-] [H2O][H2O]
[ ] = concentration in mol L-1
Water is in huge excess so we leave it out of the equation
Call it Kw not Kc

4. [H3O+] = 1 * 10-7 = 1 * 10-14 Kw = [H3O+][OH-]
= 1 * 10-14
In pure water both [H3O+] & [OH-] are present in equal concentrations
so lets assign them a value of x.
[H3O+][OH-] = x * x = x2
x2 = 1 * 10-14
x = 1 * 10-7
[H3O+] = 1 * 10-7

5. pH = -log10 [H3O+]
pH of water = -log10 [1*10-7] = 7
Water is neutral because it has equal numbers of H3O+ and OH- ions
addition of acid to water increases [H3O+] but Kw remains constant for this to happen [OH-] must drop proportionately.

6. Proportion dissociated
1 dm3 [L] of water = 1000g
1 mole of water = = 18 g
1 dm3 [L] = 1000/18 = 55.5 moles
[H2O] = 55.5
[H3O+] = 1*10-7
H3O+ : H2O = 1*10-7 : 55.5
= 1 :
1 in 555 million molecules dissociated