An ionic bond forms between a metal and a nonmetal. So, since C and O are nonmetals it cannot be that one. Also, P and Cl are also both nonmetals so it cannot be that one either. That leaves Li and O and Ca and Br. In an ionic bond metals loose electrons and nonmetals gain electrons. Therefore it cannot be Li and O because it states that oxygen, the nonmetal, looses electrons and lithium, the metal, looses electrons. So, the answer has to be Ca and Br because Ca, the metal, looses electrons and Br, the nonmetal, gains electrons.
I believe you can simply add more cells in series to get a higher voltage. Not 100% sure on that.
Think of it this way: it is a distribution problem in which you are multiplying the 2 on the outside with each element in the parentheses. Oxygen does not have a number in front of it, so put an imaginary one in front of it to help you. Do the same with Hydrogen since it doesn't have a number in front of it either. Now you know that hydrogen has one ion and oxygen has one... but you must now multiply each of the elements' ions by two. You should now know that Oxygen has 2 ions in Calcium Hydroxide and that there are also 2 ions of Hydrogen in Calcium Hydroxide. Does this make sense?
