Stoichiometry 3: Finding the number of moles of water produced when 5 moles of hydrogen react with excess oxygen

Let’s clear up a question that feels too easy to be tricky

You’re given 5 moles of hydrogen and excess oxygen.

The task is to figure out how many moles of water will be formed.

Seems like a quick calculation, right?

But here’s where students often go wrong — they forget the power of the mole ratio.

This will blow your mind… excess oxygen doesn’t mean you focus on it.

Hydrogen becomes the one that limits how much water you’ll actually get.

Always start with the balanced chemical equation

2 H₂ + O₂ → 2 H₂O

This equation tells you that every 2 moles of hydrogen react with 1 mole of oxygen to form 2 moles of water.

Because oxygen is in excess, it won’t limit the reaction.

So hydrogen becomes the key player in this scenario.

Here’s what to do with your 5 moles of hydrogen

Using the balanced equation:
2 moles of H₂ produce 2 moles of H₂O

So with 5 moles of hydrogen:
(2 moles H₂O / 2 moles H₂) × 5 moles H₂ = 5 moles of H₂O

Your final answer: **5 moles of water will be produced**.

No extra oxygen affects this outcome. Once the hydrogen is gone, the reaction stops.

Why excess oxygen doesn’t change the outcome

Are you still with me? Here’s why the extra oxygen doesn’t matter.

Excess just means you have more than enough.

Since hydrogen gets used up first, it limits how much product forms.

The reaction ends when hydrogen runs out — no matter how much oxygen is floating around.

A common error to avoid at all costs

I show all my students this and they’re always amazed

many people think they need to involve excess oxygen in the calculation.

But no. You calculate based on the reactant that limits the reaction.

In this case, hydrogen is the only factor controlling the outcome.

How this applies to real-world chemistry

I know you want to know more about chemistry so let’s bring it down to earth.

In hydrogen fuel cells, engineers often flood the system with excess oxygen to ensure every bit of hydrogen reacts efficiently.

But the amount of hydrogen still determines how much energy and water the cell can produce.

In industrial chemical processes, knowing which reactant limits production helps companies avoid waste and plan accurately.

Mistakes that could trip you up

– Using oxygen quantities in calculations even when it’s excess
– Ignoring the balanced equation
– Assuming both reactants contribute equally in all scenarios
– Rushing through without verifying who limits the reaction

Know this important step and you’ll never FAIL this question again.

Let’s sum up what you’ve learned

Step 1: Always refer to the balanced chemical equation
Step 2: Identify the limiting reactant (hydrogen in this case)
Step 3: Use mole ratios to calculate product amounts
Step 4: Don’t let excess reactants distract you from the real calculation

These steps are simple but powerful when used correctly.

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