Which numbers are used? Consider the balanced chemical equation that follows. You are asked to determine how many moles of water you can produce from 4.0 mol of hydrogen and excess oxygen. (Excess oxygen means that so much oxygen is available it will not run out.) Which of the numbers that appear in the balanced chemical equation below are used to perform this calculation?

I am assuming that the balanced equation is

2H₂ + O₂ ⇒ 2H₂O

Since the hydrogen is the limiting reagent (meaning if the hydrogen runs out the reaction stops), we need to compare the mole ratio between hydrogen and water.

The mole ratio between H₂ and H₂O is 2:2, or 1:1 (as seen on the coefficients of the balanced equation)

And so for every mole of hydrogen, it will be equal to water. Since hydrogen is 4 moles, 4 moles of water would be created.

So we use the coefficients of H₂ and H₂O in order to answer this question.