Which is the heavier??

'Moles' is the weight in grams which is proportional to the molecular weight of the substance. So for Oxygen, Oxygen has an atomic weight of 16. A molecule of oxygen gas consists of two atoms so the molecular weight is 32, therefore 2 moles is 64 grams. For zinc the molecular weight of zinc is 65.4 (allowing for different isotopes) so the zinc is heavier. Now work out the rest of the answers for yourself!