Stoichiometry and the Mole Concept
Core concepts
Stoichiometry links measurable quantities (mass, volume, concentration) to particle counts through the mole. Key relationships: n = m/M, n = N/N_A, and for gases at standard conditions n = V/V_m. Avogadro's number is 6.02 × 10²³ per mole.
Balanced equations give mole ratios. Candidates must convert between units, identify the limiting reagent, and calculate theoretical and percentage yield. Concentration is c = n/V in mol dm⁻³. Empirical formulae come from percentage composition; molecular formulae require the molar mass.
Traps to watch
- Using an unbalanced equation for mole ratios.
- Mixing units (grams with dm³, or cm³ without converting to dm³ by dividing by 1000).
- Assuming the reactant with the greatest mass is in excess - always compare moles divided by stoichiometric coefficients.
- Confusing empirical and molecular formulae.
- Forgetting that percentage yield = (actual/theoretical) × 100, while atom economy compares molar masses of desired product to total reactants.
Worked example
How many moles are in 9.0 g of water? M(H₂O) = 18 g mol⁻¹, so n = 9.0/18 = 0.50 mol. Rule of thumb: always convert to moles before applying any equation ratio, and convert back only at the end.