1. Water and pH

4. Describing the dissociation of water:
By convention, we say: H2O « H+ + OH-,
where [H+] = [H3O+]
For this reaction, we can write an equilibrium constant:
Keq = [H+][OH-]/[H2O]
Because [H2O] is approximately constant, we can define Kw = Keq *[H2O]
Kw = [H+][OH-] = (10-7)( 10-7) = M2
Next, we define the p function: p[X] = –log10[X]
pKw = pH + pOH = = 14
(agrees with above, because –log(10-14) = 14)

10. H
H2O
OH- +

11. Summary:
Water dissociates to form H3O+ (= H+) and OH-
We describe the dissociation of water (and acids
and bases) with the equilibrium constant and the p
function.
The dissociation of strong acids and bases is
considered to be complete.
The pH (and pOH) of weak acids and bases can be calculated from the equilibrium constant (and
vice-versa).