moles of O atoms in 7.18 g Mg(NO3)2?

Answer 1

Ok what you need to do is find out how many moles of
Mg(NO3)2 you have. to do that divide by the molecular wieght.

so now you have moles of Mg(NO3)2
you have 2 NO3's which have 3 O's each so you have 6 moles of O per mole of Mg(NO3)2 so multiply the number of moles of Mg(NO3)2 by 6 to get the number of moles of O2