In the molecular formula, how many moles of oxygen are there? You can find the answer by multiplying #3# by #2#, so there are 6 moles of oxygen. You can do this since there is one mole of #Cu(NO_3)_2#. If there were #2# moles of #Cu(NO_3)_2# instead, you would have an answer of 12 moles, since there would be twice as many #Cu(NO_3)_2# molecules now.
As an aside, how many moles of Copper (Cu) are there? There's #1#, since Cu doesn't have a subscript.
If you're uncertain why this is the case, you can always use the mass percent concept to double check:
#((16.00 " g Oxygen")(6 " moles Oxygen"))/(187.56 " grams "Cu(NO_3)_2) * 100 = 51.18 %#
Now we can find the number of moles of oxygen in one mole of #Cu(NO_3)_2#:
#(0.5118 * 187.56 " grams " Cu(NO_3)_2) * (1 " mole Oxygen")/(16.00 " grams of Oxygen") = 6 " moles of Oxygen"#
