So I've learnt that in strong acids that dissociate completely, the concentration of H+ is the same as the concentration of the initial solution. So 1M of a strong acid will create 1M of H+, meaning the pH is 0. I've also learnt that in weak acids, the whole thing doesn't dissociate. I have to use the acid dissociation constant to calculate the concentration of H+. But in some parts of my textbook, it also says I can use Kw to calculate the pH of acids, eg. if a solution has a concentration of 10^-4 H+ ions, then the pH is 4. But wouldn't I need to first find the acid dissociation constant of that solution? In other words, when can I use Kw, and when do I have to calculate Ka? Can Kw be used only for water, or does it apply to some acids as well? Thanks a lot for your help.