For example, in the reaction
2H₂(g) + O₂(g) → 2H₂O(g)
The mole ratio between O₂ and H₂O is
The mole ratio between H₂ and H₂O is
How many moles of O₂ are required to form 5.00 moles of H₂O?
5.00 mol H₂O ×
If the question had been stated in terms of grams, you would have had to convert grams of H₂O to moles of H₂O, then moles of H₂O to moles of O₂ (as above), and finally moles of O₂ to grams of O₂.