(1000gH2O)/(18g/mole) M = In fact, the concentration of water will always be so much greater than anything else in solution that we can consider it to be a constant. Thus, we often times end up incorporating it into the equilibrium constant, K. There is however, more to water than just water. Did you know water is a weak acid AND a weak base, a switch hitter? Notice that while the molecule, H2O is in large excess, there is some dissociation taking place. H2O Hey, see the proton and hydroxide ions? This water has acid and base present in it at the same time!! We can make an equilibrium equation: [H+][OH-] K = ____________________ [H2O] but remember that [H2O] is a constant 55.4 M. This means we can take it up into the equilibrium constant and create a new constant, Kw. Note that with an equilibrium constant for water of 10-14, a little algebra can be done to show why the pH of water is 7. Example. What is the proton concentration in pure water?