Use Mole Ratios from Balanced Equations

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Learn the skill

Coefficients in a balanced equation give mole ratios, not mass ratios. Multiply a known amount by the ratio with the desired substance on top. Check which reactant runs out when both amounts are limited.

Worked example

For 2H2 + O2 -> 2H2O, 3 mol O2 can produce 6 mol H2O if hydrogen is in excess. Multiply 3 by the ratio 2 mol H2O per 1 mol O2. These are calculations only.

Theoretical Reaction

Use 2H2 + O2 -> 2H2O. Assume complete reaction, no side reactions, and sufficient amounts of any reactant described as in excess. A mole is an amount of substance containing a fixed number of particles. Do not attempt this reaction.

1. With excess O2, how many moles of water can 8 mol H2 produce? Answer

2. How many moles of O2 are needed to react completely with 10 mol H2? Answer

3. With excess H2, how many moles of water can 4 mol O2 produce? Answer

4. A mixture has 6 mol H2 and 2 mol O2. Which reactant limits water production? H2 O2 Neither; both are exactly used up

5. For that mixture of 6 mol H2 and 2 mol O2, how many moles of water form? Answer