If we have an acid-base aqueous equilibrium system and decide to treat the concentration of H2O as a variable, does it then become impossible to solve for all the concentrations? Or can we write an equation to help us? e.g. HA is added to water. Mass Balance: CHA=[HA]+[A-] Charge Balance: [H3O+]=[A-]+[OH-] Equilibria: Ka=[A-]*[H3O+]/([HA]*[H2O]) Kw=[H3O+]*[OH-]/([H2O]2) I'm not sure the auto-dissociation of water is called Kw still - certainly the constant I have written will not value 10-14 at 298.15 K and 1 bar pressure - but I still used the same equilibrium constant name. Our variables are: [H3O+], [OH-], [H2O], [A-], and [HA]. 5 variables, 4 equations - any way to do this?