How many moles of H2O form when 4.5 moles O2 reacts?

Explanation:

Start with a balanced equation.

2H2 + O2 → 2H2O

Assuming that H2 is in excess, multiply the given moles H2O by the mole ratio between O2 and H2O in the balanced equation so that moles H2O cancel.

5 mol H2O × (1 mol O2/2 mol H2O) = 2.5 mol O2

2.5 mol O2 are needed to make 5 mol H2O, assuming H2 is in excess.