Chemistry: Mole Concept, Periodicity, Bonding and Equilibrium
The Chemistry section covers atomic structure, periodic trends, chemical bonding, the mole concept, equilibrium, basic organic chemistry and biomolecules. Expect a mix of factual recall and one or two stoichiometry calculations.
Core rules and concepts
- Mole concept: n = mass/molar mass; one mole contains 6.022 × 10²³ particles; one mole of any gas occupies 22.4 L at STP.
- Periodic trends: atomic radius decreases across a period and increases down a group; ionisation energy generally rises across a period, with dips at Be→B and N→O; electron affinity of Cl exceeds that of F.
- Bonding: count electron domains to assign VSEPR shape and hybridisation (sp, sp², sp³).
- Equilibrium: Le Chatelier's principle predicts shifts; pH = −log[H⁺]; Kw = 10⁻¹⁴ at 25 °C.
- Biomolecules: glucose is an aldohexose; amino acids are zwitterionic; DNA bases pair A–T and G–C.
Worked example
For 2H₂ + O₂ → 2H₂O, mix 4 g H₂ (2 mol) with 16 g O₂ (0.5 mol). The balanced equation needs 1 mol O₂ for every 2 mol H₂, so 0.5 mol O₂ consumes only 1 mol H₂. Oxygen is the limiting reagent; water formed = 2 × 0.5 = 1 mol = 18 g, and 1 mol H₂ (2 g) remains unreacted.
Traps that cost marks
- Using 16 g/mol for O₂ instead of 32 g/mol.
- Forgetting that pH is logarithmic: a tenfold dilution of a strong acid raises pH by 1, not by 10.
- Applying periodic trends without the Be/B and N/O exceptions.
- Confusing empirical and molecular formulae.
- Assigning oxygen an oxidation state of −2 in peroxides, where it is −1.