The formula
n = m / M
| Symbol | Quantity | Unit |
|---|---|---|
| n | Amount of substance | mol |
| m | Mass | g |
| M | Molar mass | g/mol |
What it means
The molar mass is the mass of one mole of a substance, in grams, and it is numerically equal to the atomic or molecular mass. Dividing a mass by it gives the number of moles, which is what stoichiometry needs.
How to work it out by hand
- Find the molar mass by adding the atomic masses
- Divide the mass in grams by the molar mass
- The result is the number of moles
What is worth knowing
The mole exists because reactions happen between particles in whole-number ratios, and you cannot weigh out particles. Water's molar mass of 18 g/mol means 18 grams of water contains as many molecules as 12 grams of carbon-12 contains atoms — that equivalence is the point of the definition. Since 2019 the mole has been defined by fixing Avogadro's number exactly, rather than by reference to carbon, which removed the last physical artefact from that corner of the SI.