Water and pH

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) = 10-14 M2 Next, we define the p function: p[X] = –log10[X] pKw = pH + pOH = 7 + 7 = 14 (agrees with above, because –log(10-14) = 14)

H H2O OH- +

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).