[DustinByfuglien]

I've been stumped on this one for a while now and I can't seem to find an answer anywhere. That leads me to believe that I'm totally missing something.

I need to know why/how we can calculate OH- concentration for a reaction that does not include OH-.

For example if 0.077M of HCl dissociates in water.

HCl ---> H+ + Cl-

I know that since this is a strong acid, the concentration of the H+ will equal the concentration of the acid. My question is how can we use the Kw equation (Kw = [H+][OH-]) to find OH- when it is not present in the reaction equation.

Thanks in advance!

[Borek]

In water there is always a reaction

H₂O H⁺ + OH^-

taking place. So even if the acid dissociation reaction doesn't change concentration of OH^- directly, it does that through a reaction that occurs next.