Chemistry - Mie Entrance Science
The Chemistry section covers atomic structure and periodicity, chemical bonding, stoichiometry, equilibrium, acids and bases, redox, and introductory organic chemistry. The exam favors quantitative reasoning: mole calculations, equilibrium shifts, and pH.
Core rules
- Mole bridge: n = m/M, c = n/V, and gas volume at STP is 22.4 L mol⁻¹ (use the value stated in the paper).
- Limiting reagent: divide moles by stoichiometric coefficients; the smallest quotient limits.
- Periodicity: atomic radius decreases across a period and increases down a group; ionization energy generally increases across a period.
- Equilibrium: changing concentration, pressure, or temperature shifts position, not the equilibrium constant - except temperature. Catalysts change rate only.
- Acids and bases: strong acids dissociate completely; weak acids require Ka. For weak acid HA, Ka ≈ x²/(c − x).
- Redox: balance atoms, then charge; oxidation-number changes identify oxidation and reduction.
Worked example
Find the pH of 0.10 M acetic acid with Ka = 1.8 × 10⁻⁵. Let x = [H⁺]. Then 1.8 × 10⁻⁵ = x²/(0.10 − x) ≈ x²/0.10. So x = √(1.8 × 10⁻⁶) = 1.34 × 10⁻³ M, and pH = −log(1.34 × 10⁻³) = 2.87. The approximation is valid because x is less than 5% of 0.10 M.
Traps
- Using the initial concentration as [H⁺] for a weak acid.
- Reporting pH with too many decimal places; pH decimal places match significant figures in concentration.
- Confusing “equilibrium shifts right” with “K increases”.
- Including spectator ions in a net ionic equation.
- Mixing oxidation and reduction: oxidation number increases on oxidation.