Final answer:
In the reaction of 4 moles of hydrogen with 5 moles of oxygen to produce water, the balanced chemical equation shows that hydrogen and oxygen react in a 2:1 mole ratio. Since only 2 moles of oxygen are needed for 4 moles of hydrogen, oxygen is the excess reagent.
Step-by-step explanation:
When 4 moles of hydrogen react with 5 moles of oxygen to produce water (2 H₂ + O₂ → 2 H₂O), the balanced chemical equation tells us that 2 moles of hydrogen react with 1 mole of oxygen. To determine the excess reagent, we examine the mole ratio from the balanced equation and compare it to the amounts of reactants provided.
If we use 4 moles of hydrogen, according to the balanced equation, we would need 2 moles of oxygen (since the ratio is 2:1). Given that we have 5 moles of oxygen available, only 2 moles are actually needed to react with the 4 moles of hydrogen. This means that we have excess oxygen. Therefore, the excess reagent in this reaction is oxygen (b. Oxygen).