Chemistry: Theoretical, Inorganic and Organic
Chemistry questions test quantitative reasoning alongside descriptive knowledge: mole calculations and equilibrium, periodic trends and inorganic reactions, and organic structures and isomerism. Answers are marked for balanced equations, correct units and explicit reasoning.
Core rules
- Amount of substance n = m/M; concentration c = n/V. Identify the limiting reagent before calculating yield.
- Periodic trends: atomic radius decreases across a period and increases down a group; ionisation energy generally rises across a period.
- Equilibrium: K depends on temperature only; Le Chatelier's principle predicts the direction of shift, not a new K value.
- Acids and bases: pH = −log[H⁺]; for a weak acid HA, Ka ≈ x²/C when dissociation is small.
- Organic chemistry: classify by functional group, then check chain, position and geometric isomerism; substitution patterns on a benzene ring follow the directing effects of the existing group.
Worked example
Find the pH of 0.10 mol/L weak acid HA with Ka = 1.0 × 10⁻⁵. Let x = [H⁺]. Then Ka = x²/(0.10 − x) ≈ x²/0.10, so x² = 1.0 × 10⁻⁶ and x = 1.0 × 10⁻³ mol/L. pH = −log(1.0 × 10⁻³) = 3.00. The approximation is valid because x/0.10 = 1%, well below the usual 5% test.
Traps
Unbalanced equations, mixing Ka with pKa, treating a weak acid as fully dissociated, and ignoring dilution after mixing two solutions are frequent errors. In organic answers, a correct formula with the wrong structural isomer earns little credit; draw the skeleton and number positions.