Foundation reading 6 of 17 · ~7 min

Connect grams, moles and particles

Your goal: Follow unit cancellation through mass, amount and particle-count conversions.

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

Optional prerequisite review

Review if useful. These suggestions never lock a step.

The conversion map: mass, moles, particles

Almost every mole problem is a walk along this two-step map. Molar mass connects grams and moles; the Avogadro constant connects moles and particles. Moles sit in the middle, so a grams-to-particles question is never one step.

Write each conversion as a fraction whose bottom unit cancels the unit you already have; if the units cancel to leave the unit you want, the setup is right.

grams ÷ M → MOLES × N_A → particles (reverse: × M and ÷ N_A)

Unit cancellation is your error check. If you end up with g·mol instead of mol, you multiplied where you should have divided.

Counting atoms inside a formula

The subscripts in a formula are themselves mole ratios. One mole of Al₂(SO₄)₃ contains 2 mol of Al atoms, 3 mol of S atoms and 12 mol of O atoms.

That extra ratio is a third conversion step, applied after you reach moles of the compound: multiply by the subscript to get moles of the element, then by N_A if the question asks for atoms.

For ionic compounds, the same idea gives moles of ions: 1 mol of Mg₃(PO₄)₂ contains 3 mol Mg²⁺ ions and 2 mol PO₄³⁻ ions.

1 mol Al₂(SO₄)₃ → 2 mol Al, 3 mol S, 12 mol O

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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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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