The mole as a bridge between scales
The amount of substance n is measured in moles. One mole contains the Avogadro constant, NA = 6.022 x 1023 mol-1, of specified entities. Always name the entity: atoms, molecules, ions, electrons or formula units.
Molar mass and dimensional analysis
Molar mass M has units g mol-1. In n = m/M, units cancel to mol. Carry units through every line: they expose many setup errors. Chemical coefficients give mole ratios, not mass ratios. Balance the equation before using them.
Gas quantities and conditions
Molar gas volume is not a universal constant. For an ideal gas, Vm = RT/P; it changes with temperature and pressure. A quoted value such as 22.4 dm3 mol-1 is valid only at its specified conditions. For general conditions use PV = nRT with compatible units and temperature in kelvin.
Stoichiometric workflow
- Write and balance the chemical equation.
- Convert the known quantity to moles.
- Use the coefficient ratio to obtain moles of the target.
- Convert to the requested mass, volume, particles or concentration.
- Check units, significant figures and whether the result is physically plausible.
Limiting reagent, yield and purity
When two reactants are given, calculate how much product each could form; the smaller amount identifies the limiting reagent. Theoretical yield follows from it. Percentage yield = actual yield/theoretical yield x 100%. If a sample is impure, reactive mass = sample mass x purity fraction.
| Quantity | Relation | Unit check |
|---|---|---|
| Particles | N = nNA | mol x mol-1 |
| Mass | m = nM | mol x g mol-1 |
| Solution | n = cV | mol dm-3 x dm3 |
| Ideal gas | PV = nRT | Use a matched value of R |