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

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

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

[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

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.

Proportion dissociated 1 dm3 [L] of water = 1000g 1 mole of water = 1+1+16 = 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 : 555 000 000 1 in 555 million molecules dissociated