Chemistry / Unit 3 Chem 0376 worked example, 60 to 90 seconds
Converting Mass to Moles
The idea
Converting a sample's mass to a mole amount means dividing the measured mass by the substance's molar mass, letting grams cancel against grams per mole.
In practice
A sample of potassium weighs 4.7 g. Potassium's molar mass is 39.10 g/mol, so dividing 4.7 g by 39.10 g/mol gives about 0.12 mol of potassium. The result is reasonable: 4.7 g is a bit more than one-tenth of 39.10 g, so an amount over 0.10 mol makes sense before the division finishes, a useful check on the final answer before it is trusted. The same setup works for any element or compound once its molar mass is known, so this single division pattern covers every mass-to-mole conversion a chemist will ever need to perform.
Where people slip
Dividing molar mass by sample mass instead of the reverse inverts the units, producing a reciprocal-mole quantity that looks plausible but means nothing chemically.
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