1. The problem statement, all variables and given/known data What are the number of ionizable hydrogen atoms in phosphorus acid H3PO3? 2. Relevant equations - 3. The attempt at a solution I didn't even know what "ionizable " means. ( This is not a homework questions, I'm solving sample questions for an entrance exam) . So I searched the web . But still I don't know how to decide the number of ionizable hydrogen atoms in H3PO3 . Please help.