Foundation reading 12 of 17 · ~7 min

Connect reactant mass to product mass

Your goal: Follow mass → moles → mole ratio → mass and round only at the end.

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.

The stoichiometry road map

Almost every problem in this lesson follows one path: mass of known → moles of known (divide by M) → moles of unknown (multiply by the mole ratio) → mass of unknown (multiply by M). Draw the road map before you calculate.

Knowing which arrow you are on prevents the classic error of multiplying when you should divide.

munknown = (m_known / M_known) × (coeffunknown / coeff_known) x Munknown

Significant figures and rounding

Coefficients are exact counting numbers and never limit significant figures, and molar masses are ordinarily supplied to enough digits that they do not limit the reported result. The measured quantity in the problem sets the precision.

Carry extra digits through intermediate steps and round only at the end, otherwise a three-step calculation can drift in the last digit.

Rounding at each intermediate step is the most common source of near-miss answers.

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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