Guides
Converting Moles, Grams, Liters and Particles (with formulas)
The mole bridges what you can measure (mass, volume) and what you cannot see (particle count). Almost every chemistry calculation first converts the known quantity to moles, then to the wanted quantity. Three conversion formulas cover most cases.
Three core formulas
- Mass ↔ moles: n = m ÷ M (M is molar mass, g/mol).
- Particles ↔ moles: N = n × Nᴀ, where Nᴀ = 6.022×10²³ /mol (Avogadro's number).
- Gas volume ↔ moles: at STP (0 °C, 101.325 kPa) Vm ≈ 22.4 L/mol, so n = V ÷ 22.4 (L); otherwise use PV=nRT.
Steps
- Convert to moles Turn the known quantity (mass, volume or particle count) into moles with the matching formula.
- Bridge with ratios If a reaction is involved, use the balanced coefficient ratio to go from reactant moles to product moles.
- Convert to the target unit Convert moles into the mass, volume or particle count the question asks for.
Worked examples
| Problem | Calculation | Result |
|---|---|---|
| Moles in 36 g water | 36 ÷ 18.02 | ≈ 2.0 mol |
| Volume of 2 mol O₂ at STP | 2 × 22.4 | 44.8 L |
| Molecules in 0.5 mol | 0.5 × 6.022×10²³ | ≈ 3.01×10²³ |
Tips
- 22.4 L/mol applies only to a gas at STP; for other conditions use the ideal gas law.
- Molar mass equals the relative molecular mass numerically — see how to calculate molar mass.