01 / Understand the relationship

What runs out first?

A limiting reactant sets the most product a reaction can make. Compare what each starting amount could produce.

Today’s question

If hydrogen and oxygen are supplied in different amounts, which one sets the water yield?

02 / See one workedRead at your pace

Start with the ratio.

The coefficients say two moles of hydrogen pair with one mole of oxygen to form two moles of water.

2H₂ + O₂ → 2H₂O

Suppose you begin with 6 mol H₂ and 2 mol O₂. Calculate a possible water yield from each supply.

From 6 mol H₂6 molH₂O possible

From 2 mol O₂4 molH₂O possible

Oxygen limits the reaction: 4 mol H₂O form. That uses 4 mol H₂, leaving 2 mol H₂.

These theoretical yields assume a complete reaction, with no side reactions or collection losses.

03 / Use the ideaNew amounts

Now you try.

Start with 4 mol H₂ and 3 mol O₂. How many moles of water can form?

2H₂ + O₂ → 2H₂O

Assistance: independent

Check your reasoning

Which reactant limits this case? Say why, then compare with the answer when you are ready. This reasoning is for your own review; only the number is checked.

Your reasoning (optional)

Without JavaScript: 4 mol H₂O form. Hydrogen is limiting because it could make 4 mol water, while oxygen could make 6 mol. The reaction uses 2 mol O₂, leaving 1 mol O₂.

If you want to go further

Take it away from the page.

Tomorrow, redraw the ratio from memory and explain why the smaller possible yield decides the product amount. Delayed retest: not yet checked.

Source

Trace the idea.

Concept: OpenStax Chemistry 2e, §4.4 Reaction Yields. The examples and wording here are original.