[Decoherence]

I need some help with an AP Chem lab. If you have an unknown aqueous copper solution, and you add magnesium to it to make the copper precipitate out, how can you find the unknown part of the solution? And do you even need to in order to calculate the percent copper and formula mass of the copper? I would assume it needs to be a -2 polyatomic ion, but if it is important for the problem, how should I find out which?

Cu(II)(?) + Mg = Mg(?) + Cu