Chemistry: Moles, Equilibrium, Acids–Bases and Functional Groups
Scope
The chemistry section tests numerical fluency and structure recognition: moles and stoichiometry, atomic structure and periodic trends, bonding, equilibrium, acids and bases, redox, and the functional groups that matter in biochemistry.
Core rules
- Mole: n = m/M; 1 mol contains 6.022 × 10²³ particles; at STP one mole of ideal gas occupies 22.4 L; PV = nRT.
- Solutions: molarity uses litres of solution, not of solvent; dilution follows M₁V₁ = M₂V₂.
- Equilibrium: a catalyst changes rate only and does not shift position. Le Chatelier predicts the direction of shift after a change in concentration, pressure or temperature.
- Acids and bases: Kw = 10⁻¹⁴ at 25 °C; pH = −log[H⁺]; a 10-fold dilution of a strong acid raises pH by one unit.
- Organic: alcohols oxidise to aldehydes and then carboxylic acids; carboxylic acid plus alcohol gives an ester; amines are basic.
Worked example with reasoning
4.0 g of NaOH (M = 40 g/mol) is dissolved and made up to 500 mL. Moles = 4.0/40 = 0.10 mol. Molarity = 0.10/0.500 = 0.20 mol/L. NaOH is a strong base, so [OH⁻] = 0.20 M, pOH = −log(0.20) = 0.70, and pH = 14.00 − 0.70 = 13.30. The volume used is the final solution volume, not the water added.
Traps that cost marks
- Using the mass of water instead of the final volume.
- Forgetting that a weak acid needs Ka before pH can be found.
- Assigning oxygen −2 in peroxides, where it is −1.
- Balancing redox half-reactions without checking atoms and charge in acidic or basic medium.
Practice that transfers
Convert every problem into moles first and write units on each line; most lost marks are unit or volume errors rather than chemistry errors.