Use Coefficients as Mole Ratios

Supplied materials

• Use the supplied fictional observations or model. Paper and pencil are optional; spoken answers are welcome. No equipment or real experiment is needed.

Learn the idea

Coefficients in a balanced equation give ratios of particles, and therefore ratios of moles. A mole is a counting unit containing a fixed very large number of entities. A ratio of 2 molecules to 1 molecule also gives 2 mol to 1 mol. Coefficients are not gram ratios; different substances have different mass per mole.

Use the paper equation 2 H₂ + O₂ → 2 H₂O. For this calculation assume the reaction goes to completion as represented and enough O₂ is supplied. Each 2 mol H₂ used produces 2 mol H₂O and uses 1 mol O₂. If 6 mol H₂ reacts, scaling by three gives 3 mol O₂ required and 6 mol H₂O produced. No physical reaction is part of this lesson.

1. Write the coefficient ratio H₂:O₂:H₂O as 2:1:2.
2. Divide 6 mol H₂ by the 2 mol ratio entry to obtain a scale factor of 3.
3. Multiply each other ratio entry by 3.
4. Check that mol units remain and explain why the same numbers are not gram ratios.

Check your reasoning

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Take it further

Change to N₂ + 3 H₂ → 2 NH₃. With sufficient H₂, 4 mol N₂ requires 12 mol H₂ and produces 8 mol NH₃ under the same completion model.

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