Foundation reading 11 of 17 · ~6 min

Read a reaction's mole ratio

Your goal: Use coefficients as mole ratios and orient a conversion so units cancel.

A focused reading from Mole Ratios and Mass-to-Mass Stoichiometry. Reading time is a Chem Help estimate.

Optional prerequisite review

Review if useful. These suggestions never lock a step.

Coefficients are mole ratios

For 2 H₂ + O₂ → 2 H₂O, the coefficients say that 2 mol H₂ reacts with 1 mol O₂ to make 2 mol H₂O. They do not say 2 g reacts with 1 g.

Coefficients count particles, and only moles count particles, so any ratio you take from an equation must be applied to moles.

2 H₂ + O₂ → 2 H₂O

Coefficients are mole ratios, never mass ratios.

Writing the ratio the right way up

Put the known substance on the bottom of the ratio fraction so its unit cancels. Converting 5.0 mol H₂ to water uses (2 mol H₂O / 2 mol H₂). If your units do not cancel, the fraction is upside down.

This single check catches most wrong answers.

Write every conversion as a fraction and cross out units as you go.

Sources and further reading

  • Chemistry 2e, Section 4.3: Reaction Stoichiometry
    Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson · OpenStax, Rice University · Chapter 4.3
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    Lesson-specific rights and provenance need review.

    The current online Chemistry 2e edition lists CC BY-NC-SA 4.0. The source version used for this lesson and its applicable rights remain unverified.

    Recorded use: reference. This describes the stored citation, not verified authorship or reuse permission.

    Recorded attribution / license claim (applicability unverified): Chemistry 2e, OpenStax, Rice University, licensed CC BY 4.0.

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  • General Chemistry bookshelf (LibreTexts)
    LibreTexts, University of California, Davis
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    Lesson-specific rights and provenance need review.

    The exact LibreTexts page and its license have not been established; the bookshelf citation does not grant permission for all linked material.

    Recorded use: further reading. This describes the stored citation, not verified authorship or reuse permission.

    Recorded attribution / license claim (applicability unverified): LibreTexts General Chemistry, licence varies by page, check the page footer.

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