Why is the concentration of water constant in acid-base equilibria?
Kw is an adjusted version of Kc, where the concentration of water is constant. But I dont understand why they take it to be constant. Why aren't the concentrations of OH- and H+ constant as well 
Original post by lorobolorolo
Kw is an adjusted version of Kc, where the concentration of water is constant. But I dont understand why they take it to be constant. Why aren't the concentrations of OH- and H+ constant as well
Because water dissociates giving an equilibrium between water molecules and hydrogen and hydroxide ions:
H2O <==> H+ + OH-
as this is an equilibrium it is governed by the equilibrium law:
kc = [H+][OH-]/[H2O]
The water is in HUGE excess and it's concentration can be taken as the initial (undissociated) concentration. This is to all intents and purposes constant, so a new constant is defined that combines water and kc:
kc*[H2O] = kw = [H+][OH-]
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