General and Inorganic Chemistry
General and Inorganic Chemistry tests quantitative reasoning: moles and limiting reagents, gas laws, equilibrium, acid–base calculations, redox and periodic trends. Most items are numerical, so units and significant figures carry marks.
Core rules
- n = m/M; molarity = mol/L; identify the limiting reagent in moles (never in grams); percentage yield compares actual with theoretical product.
- Gases: PV = nRT with T in kelvin; 1 mol occupies 22.4 L at 0 °C and 1 atm.
- Equilibrium: K depends only on temperature; Le Chatelier predicts the direction of shift, not the new value.
- pH = −log[H⁺]. Strong acids dissociate completely; for a weak acid [H⁺] ≈ √(Ka·C); buffers obey pH = pKa + log([A⁻]/[HA]).
- Redox: track oxidation numbers; in acidic solution MnO₄⁻ → Mn²⁺ (5 electrons) and Cr₂O₇²⁻ → 2 Cr³⁺ (6 electrons).
- Periodicity: atomic radius increases down a group and decreases across a period; ionisation energy moves oppositely.
Worked example
Find the pH of 0.10 M acetic acid, Ka = 1.8 × 10⁻⁵. Acetic acid is weak, so [H⁺] ≈ √(Ka·C) = √(1.8 × 10⁻⁶) ≈ 1.34 × 10⁻³ M. pH = −log(1.34 × 10⁻³) ≈ 2.87. Check the approximation: 1.3% dissociation, well under 5%, so it holds.
Traps
- Calling 10⁻⁸ M HCl neutral or basic; water autoionisation sets a floor near pH 7.
- Using the strong-acid formula on a weak acid and reporting pH = 1 for 0.1 M.
- Reporting pH with more decimal places than the concentration's significant figures justify.
- Forgetting that Ka rises as pKa falls, and that a lower pKa means a stronger acid.