Moles & Concentration
Convert between mass and moles, or work out how much substance to weigh out for a solution of a given concentration and volume — and every reverse question.
What is a Mole?
A mole is a counting unit, like a dozen — but for 6.022×1023 particles. Chemists use it because atoms are far too small to count one by one, while a mole of a substance is an amount you can weigh.
The molar mass converts grams into moles, and the concentration (molarity) tells you how many moles are dissolved per litre of solution. Together they let you prepare any solution from a solid.
Conversion Path
m → nConcentration Units
| Unit | Meaning |
|---|---|
| 1 M = 1 mol/L | 1000 mol/m³ |
| 1 mM = 1 mmol/L | 10−3 M |
| 1 µM = 1 µmol/L | 10−6 M |
| Mass concentration | g/L = c · M |
| ppm (dilute, in water) | ≈ mg/L |
| % (w/v) | g per 100 mL |
Worked Techniques
Divide the mass by the molar mass. Use the molar mass of the exact form you weigh, including any water of crystallisation.
Concentration times volume gives the moles; times the molar mass gives the grams to weigh out.
Dissolve the solid in less solvent than needed, transfer it to a volumetric flask, then fill up to the mark and mix.
Sources & Verification
- S. S. Zumdahl & S. A. Zumdahl, Chemistry, 10th ed., Cengage, 2018 — Ch. 3 (Stoichiometry) and Ch. 4 (Types of Chemical Reactions and Solution Stoichiometry).