How do you calculate the number of moles?
Short answer
Number of moles = mass ÷ molar mass, written n = m ÷ M. Use the mass in grams and the molar mass in g/mol, which you get by adding the relative atomic masses from the periodic table. For example, 36 g of water (M = 18 g/mol) is 36 ÷ 18 = 2 mol.
Step by step
- Write the formula of the substance.
- Find its molar mass by adding the relative atomic masses of every atom in the formula.
- Make sure the mass is in grams. Convert kilograms by multiplying by 1000.
- Divide the mass by the molar mass to get moles.
Worked example
How many moles are in 10 g of calcium carbonate, CaCO₃?
- M = 40 + 12 + (3 × 16) = 100 g/mol
- n = 10 ÷ 100
- n = 0.1 mol
Common mistakes
- Forgetting to multiply atomic mass by the small number in the formula, for example O₃.
- Using the atomic number instead of the relative atomic mass.
- Mixing up the rearrangements. Mass = moles × molar mass.
Useful for:GCSEA LevelWAECMatricBac
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