Chemistry
Scope
Expect atomic structure and isotopes, electronic configuration, periodic table trends, chemical bonding, formulae and equations, the mole concept and stoichiometry, acids/bases/salts and pH, redox and oxidation numbers, gases, solutions and basic organic chemistry (functional groups, hydrocarbons, isomerism).
The three anchors
First, atomic structure: protons define the element, neutrons define the isotope, electrons define chemistry. Second, bonding: ionic bonds transfer electrons between metal and non-metal; covalent bonds share electrons between non-metals; metallic bonding explains conductivity and malleability. Third, the mole: one mole is 6.02 × 10²³ particles and equals the molar mass in grams. Almost every numerical question is a mole question in disguise.
Balance equations before calculating anything. Then convert what you are given into moles, apply the coefficient ratio from the balanced equation, and convert back to grams, litres or concentration.
Traps
Using the unbalanced equation's ratios. Treating pH as linear - a change from pH 3 to pH 5 is a hundredfold change in hydrogen ion concentration. Forgetting that the limiting reagent decides the yield. Assuming oxygen is always −2 (in peroxides it is −1). Confusing empirical and molecular formulae. Mixing up oxidation (loss of electrons) and reduction (gain).
Worked example
What is the molarity of a solution containing 4 g of NaOH in 500 cm³? Molar mass NaOH = 40 g/mol, so moles = 4/40 = 0.1 mol. Volume = 0.5 dm³. Molarity = 0.1 ÷ 0.5 = 0.2 mol/dm³.
Rule of thumb
If a question contains grams, litres or "how much", write the balanced equation first - it costs ten seconds and prevents most errors.