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Moles and Molar Mass

▶︎  Watch it animatedinteractive step-through · ~3 min · optional

Chemistry counts particles, but the lab weighs grams. The mole is the bridge between the two. One mole is Avogadro's number of anything — 6.022×10²³ particles — and the molar mass, in grams per mole, is the mass of one mole of a substance, read straight off the periodic table. With just those two constants you can travel in any direction: grams to moles by dividing by molar mass, moles to particles by multiplying by Avogadro's number, and back again the other way.

UNIT 1 TOPIC 1.1 • MOLES AND MOLAR MASS THE MOLE BRIDGE GRAMS MOLES PARTICLES 18.02 g H₂O 1.00 mol H₂O 6.022×10²³ molecules n = m / M moles = mass ÷ molar mass grams → moles: ÷ molar mass molar mass = 18.02 g/mol moles → grams: × molar mass moles → particles: × Avogadro's number Avogadro's number = 6.022×10²³ particles → moles: ÷ Avogadro's number ! CED ANCHOR The numerical value of an average atomic mass (amu) equals the numerical value of a molar mass (g/mol). Oxygen: O = 16.00 amu = 16.00 g/mol Water: H₂O = 18.02 amu = 18.02 g/mol SUBSTANCE: WATER FORMULA: H₂O MOLAR MASS 18.02 g/mol AVOGADRO'S NUMBER 6.022×10²³ AP Chemistry · Unit 1 · Atomic Structure & Properties
The mole bridge for water. Grams cross to moles by dividing by the molar mass (18.02 g/mol); moles cross to particles by multiplying by Avogadro's number (6.022×10²³). The numerical value of the average atomic mass in amu is the same as the molar mass in g/mol, which is why both sit at 18.02.
The Mole Bridge · Open the sandbox →

None of the common mistakes here are really arithmetic. They come from treating a conversion as if it changed how much substance is present, from assuming equal masses of two substances hold equal numbers of particles, and from reading a mass percent as if it counted atoms. Each one is a slip in what the number means, not in how it is computed.

The work

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Lesson
Moles and Molar Mass ›

The mole ties grams, moles, and particles together through molar mass and Avogadro's number. The lesson walks the ways students misread that bridge, then closes with a ten-scenario applet: convert between grams, moles, and particles in either direction and say what each number counts.

Skill check · 10 scenarios
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Diagnostic
10-item topic check ›

Ten items spanning the three foundation Topic 1.1 misconceptions: a conversion changing the amount of substance, equal grams read as equal particle counts, and a mass percent read as a headcount of atoms. Take it cold to surface which ones are still tangled, or after the lesson to confirm they aren't.

Not started · 10 items · ~15 min
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Targeted Practice
Drill a single misconception ›

Pick one of the failure modes you missed and drill it on its own. The round is adaptive: two correct in a row clears the misconception and moves you to the next.

Take the diagnostic to identify your misconceptions
Diagnostic first