Stoichiometry and the Mole Concept
What you must know
Be confident converting between mass, moles, particles and gas volume (molar volume at STP). Balance equations and use mole ratios to find limiting reagents, theoretical and percentage yield, and empirical versus molecular formulae. Handle concentration (mol dm⁻³), dilution, and titration calculations. Know Avogadro's number and the molar mass concept, and apply them to solutions and gases.
Traps
- Using the wrong molar volume (22.4 dm³ mol⁻¹ at STP, 24 dm³ mol⁻¹ at room conditions).
- Forgetting to balance the equation before applying mole ratios.
- Mixing units: cm³ to dm³ requires dividing by 1000.
- Confusing empirical and molecular formula when a molar mass is given.
Worked example
4.0 g of CaCO₃ (M = 100 g mol⁻¹) reacts with excess HCl. Moles CaCO₃ = 4.0/100 = 0.040 mol. From CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O, moles CO₂ = 0.040 mol, volume at STP = 0.040 × 22.4 = 0.896 dm³. Rule of thumb: always convert to moles first, then apply the balanced ratio, then convert back to the requested quantity.