AP Chemistry units guide: All 9 units, exam weightings, and where to focus your revision

AP (Advanced Placement) Chemistry is built around nine units, and the College Board publishes the exam weighting range for each. That matters for revision planning: A unit worth 18 to 22 percent of the paper is worth spending real time on. A unit worth 7 to 9 percent isn't. This guide walks each of the nine units, what's in it, roughly how much it's worth, and where students most often lose marks.

The weightings below come from the AP Chemistry Course and Exam Description (CED). They apply to the multiple-choice section; the free-response section pulls from the same content but doesn't publish per-unit percentages. If you want each unit taught with lessons, worked examples, and past-paper questions grouped by topic, Cognito's AP Chemistry course mirrors the CED unit-by-unit.

Covers moles, molar mass, mass spectrometry, isotope abundance calculations, electron configurations, photoelectron spectroscopy, and periodic trends. This is the foundation the rest of the course builds on, and although the exam weighting looks small, the ideas turn up in questions across every other unit.

Where students lose marks: Misreading photoelectron spectra, forgetting that binding energy decreases as subshell energy increases (core electrons in the lowest-energy subshells have the highest binding energy, so the 1s peak sits at the far left of a PES plot), and getting electron configurations wrong for transition metals (chromium and copper are the classic exceptions, with 4s^1 rather than 4s^2).

Ionic versus covalent bonding, Lewis structures, VSEPR (Valence Shell Electron Pair Repulsion) geometry, hybridization, and formal charge. Also covers metallic bonding and the connection between structure and macroscopic properties.

Where students lose marks: Drawing Lewis structures with the wrong number of electrons, missing lone pairs on the central atom when assigning geometry, and confusing electron geometry (based on all electron groups) with molecular geometry (based only on bonded atoms).

The heaviest-weighted unit on the exam. Covers intermolecular forces (London dispersion, dipole-dipole, hydrogen bonding, ion-dipole), states of matter, gas laws (PV = nRT, Dalton's law of partial pressures, Graham's law of effusion), solution chemistry, and separation techniques (chromatography, distillation).

Because it's worth around one-fifth of the multiple-choice section, it's the single best use of extra revision time. Questions typically ask you to rank compounds by boiling point, predict solubility, or work with gas mixtures. Ranking questions on intermolecular forces come up almost every year.

Where students lose marks: Assuming hydrogen bonding requires just any H-bond (it requires H bonded to N, O, or F), and confusing London dispersion strength (which increases with electron count and molecular size) with molecular mass alone.

Balancing equations, net ionic equations, stoichiometry, limiting reactants, percent yield, precipitation reactions, acid-base reactions, and redox reactions at an introductory level.

Where students lose marks: Writing full molecular equations when the question asks for net ionic, missing spectator ions, and stoichiometry errors when the balanced equation has fractional coefficients.

Rate laws (from experimental data, not stoichiometry), integrated rate laws (first, second, zero order), half-life, Arrhenius equation and activation energy, reaction mechanisms and rate-determining step, catalysis.

Where students lose marks: Assuming the rate law follows the balanced equation coefficients, misidentifying reaction order from graphs (ln[A] versus time for first order, 1/[A] versus time for second order), and forgetting that a catalyst lowers activation energy but doesn't change ΔH.

Calorimetry (q = mcΔT), enthalpy changes for reactions, Hess's law, bond enthalpies, standard enthalpies of formation, heating and cooling curves, phase changes.

Where students lose marks: Sign errors in calorimetry (heat lost by the reaction equals heat gained by the surroundings), forgetting to flip the sign of ΔH when reversing a reaction in Hess's law, and confusing bond enthalpy method (bonds broken minus bonds formed) with the direction convention for ΔH of formation. All the enthalpy and calorimetry equations for this unit are on the AP Chemistry equation sheet, but knowing when to apply each one is what separates high scores from low.

Kc and Kp expressions, reaction quotient Q versus K to predict direction, ICE (Initial-Change-Equilibrium) tables, Le Chatelier's principle, solubility product Ksp and common ion effect.

Where students lose marks: Getting the Kc expression wrong by leaving in pure solids or liquids, using an ICE table without checking that the assumption (x is small) is valid, and misapplying Le Chatelier to a change in a catalyst (a catalyst shifts nothing at equilibrium).

The second-heaviest unit. Covers pH and pOH, weak acid and weak base equilibria, buffer calculations (Henderson-Hasselbalch), titration curves for strong-strong, strong-weak, and weak-strong titrations, indicator selection, polyprotic acids.

Buffer calculations and titration curves appear on almost every free-response section. Being fluent with weak acid Ka calculations, buffer pH before and after adding strong acid or base, and identifying the equivalence point versus the half-equivalence point on a titration curve is worth the investment.

Where students lose marks: Using [H+] = Ka for a weak acid problem (that only works when [HA] = [A-], the half-equivalence point), confusing endpoint with equivalence point, and losing sign or log direction on pH-pKa manipulations.

Second law of thermodynamics (entropy always increases in the universe), Gibbs free energy (ΔG = ΔH - TΔS), spontaneity, ΔG = -RT ln K, galvanic (voltaic) and electrolytic cells, standard reduction potentials, Nernst equation, Faraday's laws of electrolysis.

Where students lose marks: Predicting spontaneity from ΔG without checking temperature, confusing galvanic (spontaneous, positive E) with electrolytic (non-spontaneous, requires external voltage), and forgetting that n in ΔG = -nFE is the moles of electrons transferred, not moles of anything else. Standard reduction potentials aren't in the standing reference packet – any specific E° values you need are supplied in the question stem or in an appended table, so the exam tests whether you can pull the right pairs from what's given and combine them, not memorisation.

If you divided your revision time evenly across the nine units, you'd spend roughly 11 percent of it on each. Rebalance based on weighting and you'd spend more like:

Unit 3 (intermolecular forces and gas laws): ~22 percent of your time. Unit 8 (acids and bases): ~15 percent. Each of Units 1, 2, 4, 5, 6, 7, and 9: ~9 percent each.

That's not a rigid formula – content you find easy needs less time regardless of weighting – but it's a better starting point than a flat pass. Combined with active recall (past FRQs and topic-grouped multiple-choice questions), it targets the marks you're most likely to gain. Our guide to studying for AP Chemistry shows how to build a three-month revision cycle around these weightings.

One caveat on unit weightings: They're published as ranges (7-9%, 18-22%) because the College Board rebalances slightly from year to year. Don't assume Unit 3 will always be exactly 20 percent, and don't skip a unit just because it's lightly weighted. Every unit shows up on the exam.

For a full unit-by-unit walkthrough with practice questions, Cognito's AP Chemistry course covers all nine, and the wider AP catalog has the same structure for other AP sciences.