While I am studying my Chemistry lecture notes, I find the following notes with which I am rather confused: For a particular acid and its conjugate base HA + H2O = H3O+ + A- Ka= [A-][H3O+] / [HA] A- + H2O = HA + OH- Kb=[HA][OH-]/[A-] KaKb = Kw pKw= pKa + pKb = 14 (at 298K) I am really stuck at these notes. As far as I comprehend, first of all, why there are two equilibria in the solution? With repect to the first equation, the concentration of the acid and conjugate base is already fixed. Then, how can the conjugate base react with H2O forming back HA? I think I have wrongly interpreted the information. Could anyone please kindly recitfy and guide me ? How to think about the equilibria? Why A- reacts with H2O rather than H3O (which is the right hand side of the first equation) ?