Okay, I'm reviewing how to write reactions to determine which reactant should behave as the acid and which should behave as the base. I haven't seen this material in over a year and am actually trying to help someone out with a class. I know the ionic charge should balance on both sides. 1) HF(aq) + H2O(l) <---> H3O+(aq) + F-(aq) I know that the acid is HF and its conjugate base is F-, while H2O is the base and H3O is its conjugate acid. My problem here is knowing how to write the balanced reaction. How am I supposed to know that it yields H3O+(aq) + F-(aq)? For example, I could see it going to OH- + H2F+ as well. Does the state (gas,solid,liquid, etc.) have any importance? 2) NH3(aq) +H2O(l)<--->NH4+(aq)+OH-(aq) H2O is the acid and OH- is its conjugate base. NH3 is the base and NH4+ is its conjugate acid. I'm having the same problem here. How do I know what NH3 and H2O is supposed to yield? Thanks!