Question: how many grams of oxygen gas are needed to produce 36.0 grams of water?
Answer: 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.