Which formula represents a hydronium ion?
(1) h3o+ (3) oh–
(2) nh4+ (4) hco3–

The answer is (1) H3O+. OH- is called hydroxyl. HN4+ is called ammonium. HCO3- is called bicarbonate. Hydronium ion is the combination of H+ ion(hydrogen ion) and H2O(water molecule).

1. and 2. question: in which equation? or i don't understand you, 3. a), 4. c), 5. c), 6. b), 7. d), 8. d) if small number is underlined(behind symbol) , b) if big number is underlined(in front of symbol)9. what is underlined there? 10. c)