Calculating molar mass and moles
Molar mass is the mass of one mole of a substance. Add up the atomic masses of all atoms in the formula. With it you convert between grams and moles.
Formulas and rules
- M = sum of the atomic masses of all atoms in the formula (g/mol)
- A subscript after an element counts for that element only, after a bracket for everything inside
- Water of crystallisation (e.g. · 5 H₂O) is included
- n = m / M (amount in mol)
- m = n × M (mass in g)
Your turn
Three problems on this topic. Type numbers like 12.5 or 1,250 and elasticities with their sign.
Task
Work out the molar mass of ethanol (C₂H₅OH). Atomic masses (g/mol): H 1.01 · C 12.01 · O 16.00.
Molar mass in g/mol
Numbers like 12.5 or 0.025, with as many decimals as asked. Masses and amounts count within 1%, pH values within ±0.02.
Worked example
Work out the molar mass of copper(II) sulfate pentahydrate (CuSO₄·5H₂O). Atomic masses (g/mol): H 1.01 · O 16.00 · S 32.07 · Cu 63.55.
Worked solution
- 1.Count the atoms of each element (a subscript belongs to the element before it, a bracket subscript to everything inside)
- 2.M(CuSO₄·5H₂O) = 10 × 1.01 + 9 × 16.00 + 32.07 + 63.55 = 249.72 g/mol
Typical mistake: Applying the subscript after a bracket only to the last element: Ca(OH)₂ contains 2 O and 2 H.
What amount of substance is 178.7 g of ethanol (C₂H₅OH)? Atomic masses (g/mol): H 1.01 · C 12.01 · O 16.00.
Worked solution
- 1.M(C₂H₅OH) = 6 × 1.01 + 2 × 12.01 + 16.00 = 46.08 g/mol
- 2.n = m / M = 178.7 g / 46.08 g/mol = 3.878 mol
Typical mistake: Swapping mass and molar mass (M / m instead of m / M). Check the unit: g divided by g/mol gives mol.
Common questions
Why does a mole of each substance weigh something different?
A mole always contains the same number of particles (6.022 × 10²³). The particles weigh different amounts, so a mole of water is 18 g and a mole of glucose 180 g.