Moles and molar mass
Use the mole to connect a measured mass with a chemical amount.
What you will learn
- Calculate amount from mass.
- Distinguish molar mass from relative formula mass.
Amount counts entities
The mole is a unit of amount of substance. One mole contains Avogadro's number of specified entities. The entity must be clear: atoms, molecules, ions or formula units are not interchangeable descriptions.
Molar mass is mass per mole, often in g mol⁻¹. Relative formula mass is dimensionless; its numerical value corresponds to molar mass in g mol⁻¹ at the precision used in these calculations.
Use units to check rearrangement
Dividing grams by grams per mole gives moles. If a chemical equation is involved, first calculate the amount of the known reactant, then use the balanced mole ratio, and only afterwards convert to the requested mass or other quantity.
Worked example
A sample has mass 9.0 g and molar mass 60.0 g mol⁻¹. Find its amount.
Show the worked solution
- Use n=m/M.
- Substitute 9.0/60.0.
- Report the amount with the unit mol.
Answer 0.15 mol
Common mistakes
- Do not multiply mass by molar mass.
- A balanced equation's coefficients give mole ratios, not generally mass ratios.
What can you explain now?
What mass is 0.25 mol of a substance with M=80 g mol⁻¹?
Compare with the explanation
20 g
Rearrange to m=nM=0.25×80.
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