how many grams of oxygen gas are needed to produce 36.0 grams of water?
From this we can see the molar ratios. Thus according to the molar ratio, we should have half the amount of moles, that is one mole of diatmoic oxygen. 1 Oxygen atom has a mass of 16 grams, thus diatomic oxygen weights twice as much- 32 grams. Hence 32 grams is needed.