Chemistry: Structure, Reactions and Quantitative Calculation
Core concepts to master
Know atomic structure (subatomic particles, isotopes, electronic configuration), the periodic trends of atomic radius, ionisation energy and electronegativity, and the nature of ionic, covalent and metallic bonding. Chemical equations must be balanced, and stoichiometry - mole concept, molar mass, limiting reagent, percentage yield - is the highest-yield topic. Cover acids, bases and pH, oxidation numbers and redox balancing, the gas laws, and solution concentration. Organic basics include homologous series, functional groups, and simple isomerism.
Traps examiners set
- Forgetting to balance an equation before doing mole ratios.
- Treating the reactant in excess as the limiting reagent; always compare mole ratios, not masses.
- Mixing molarity with molality, or forgetting that volume must be in litres.
- Ignoring significant figures in pH and concentration answers.
- Confusing oxidation (loss of electrons) with reduction.
Rule of thumb
Convert grams to moles, use the balanced equation as a ratio map, then convert back to the requested unit. For limiting-reagent problems, compute the moles of product each reactant would give and take the smaller.
Worked example
How many grams of water form when 4 g of hydrogen reacts with excess oxygen?
2H₂ + O₂ → 2H₂O. Moles of H₂ = 4/2 = 2 mol. Ratio H₂ : H₂O is 2 : 2, so 2 mol water forms. Mass = 2 × 18 = 36 g.
Practice habit
Write the balanced equation and the mole ratio above it before touching the calculator; most errors are setup errors, not arithmetic.