Chemistry
Chemistry tests mole calculations, atomic structure, bonding, thermodynamics, equilibrium, redox, inorganic trends and organic reaction mechanisms. The paper mixes short factual items with multi-step numerical and reasoning questions, so definitions and equations must be precise.
Core rules and concepts
- Mole concept: n = m/M; particles = n × 6.022 × 10²³; molar volume at STP is 22.4 L for an ideal gas.
- Equations: balance atoms and charge; identify the limiting reagent before calculating product yield.
- Equilibrium: apply Le Chatelier's principle and write equilibrium expressions with correct powers.
- Organic chemistry: identify functional groups, apply IUPAC numbering and distinguish substitution, addition and elimination.
- Coordination compounds: determine oxidation state, coordination number and geometry separately.
Worked example: mole calculation
4.9 g of H₂SO₄ has molar mass 98 g/mol, so n = 4.9/98 = 0.05 mol. Molecules = 0.05 × 6.022 × 10²³ = 3.011 × 10²². Because H₂SO₄ is dibasic, moles of H⁺ = 2 × 0.05 = 0.10 mol. Reasoning: convert mass to moles first, then use Avogadro's number for particles and the acid's basicity for hydrogen ions.
Traps that cost marks
- Using an unbalanced equation for mole ratios.
- Confusing molar mass with molecular mass without units.
- Treating percentage yield and percentage purity as the same quantity.
- Numbering an organic chain from the wrong end.
- Writing a coordination geometry without first finding the oxidation state.