Foundation reading 5 of 17 · ~7 min

Find the mass of one mole

Your goal: Find molar mass from atomic masses and formula subscripts.

A focused reading from The Mole and Molar Mass. Reading time is a Chem Help estimate.

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Molar mass: grams per mole

Molar mass, symbol M, is the mass of one mole of a substance, in grams per mole.

Its numerical value is the same as the atomic or formula mass in atomic mass units, which is why you can read it straight off the periodic table: carbon has an average atomic mass of 12.01 amu, so its molar mass is 12.01 g/mol.

Periodic-table values are weighted averages over the naturally occurring isotopes, so molar masses are rarely whole numbers.

M(element) = average atomic mass from the periodic table, in g/mol

Use the atomic masses printed on the periodic table you are allowed in your exam, and keep at least two decimal places until the final rounding step.

Molar mass of a compound

For a compound, add the molar mass of every atom in one formula unit. Subscripts multiply, and a subscript outside parentheses multiplies everything inside them. Setting the work out as a small table makes missing atoms obvious.

For calcium nitrate, Ca(NO₃)₂, one formula unit contains 1 Ca, 2 N and 6 O.

Ca(NO₃)₂: 1 × 40.08 + 2 × 14.01 + 6 × 16.00 = 40.08 + 28.02 + 96.00 = 164.10 g/mol

The most common molar-mass error is applying the outside subscript only to the first element inside the parentheses. In Ca(NO₃)₂ there are 6 oxygen atoms, not 3.

Sources and further reading

  • Chemistry 2e, Section 3.1: Formula Mass and the Mole Concept
    Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, et al. · OpenStax, Rice University · Chapter 3.1 · pp. Section 3.1 (web edition)
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    Recorded attribution / license claim (applicability unverified): Adapted from "Chemistry 2e" by OpenStax, Rice University, licensed CC BY 4.0. Changes were made.

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  • SI base unit: mole (mol)
    Bureau International des Poids et Mesures (BIPM) · SI base units
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    CC BY 4.0 is verified for the SI Brochure, ninth edition updated in 2026. Its applicability to this lesson's linked webpage or source version has not been established.

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  • Definitions of SI Base Units
    National Institute of Standards and Technology (NIST)
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  • Chem Help Question Bank, Unit 1 Stoichiometry, topic "Definition of the mole/Mole conversions"
    Chem Help · Chem Help

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