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If you want to do well on the AP Chemistry test, the first thing you need to do is learn all 9 units. The weight, key topics, and level of difficulty of each unit’s exam are different.
More than 168,000 students took the test in 2025, and the best students focused on the most important units.
This guide has information on all 9 units, the weights of the exams, important formulas, and a smart study plan that is based on the most recent College Board update for 2024–25.
| Where Can You Find AP Chemistry Unit Notes? | Unit 7: Equilibrium |
| What Are the AP Chemistry Units? | Unit 8: Acids and Bases |
| AP Chemistry Units — Complete Overview (2026) | Unit 9: Electrochemistry |
| Unit 1: Atomic Structure and Properties | AP Chemistry Science Practices |
| Unit 2: Molecular and Ionic Compound Structure | AP Chemistry Exam Format (2026) |
| Unit 3: Properties of Substances and Mixtures | AP Chemistry Units Ranked by Difficulty |
| Unit 4: Chemical Reactions | Most Common Mistakes on AP Chemistry |
| Unit 5: Kinetics | AP Chemistry Study Plan — Priority Guide |
| Unit 6: Thermochemistry | AP Chemistry 2024–25 CED Updates |
| Frequently Asked Questions (FAQ) | |
| Final Resources & Practice |
| AP Chemistry Unit | What’s Included | Notes |
| Unit 1: Atomic Structure & Properties | Moles, electron configuration, PES, and periodic trends, all with clear explanations of the ideas. | Notes |
| Unit 2: Molecular & Ionic Structure | Step-by-step explanations of Lewis structures, VSEPR, hybridization, and bonding ideas | Notes |
| Unit 3: Properties of Substances & Mixtures | Gas laws, intermolecular forces (IMFs), solutions, the Beer-Lambert law with examples and formulas | Notes |
| Unit 4: Chemical Reactions | Stoichiometry, net ionic equations, and redox reactions with short explanations of each concept. | Notes |
| Unit 5: Kinetics | Rate laws, the Arrhenius equation, and reaction mechanisms with easy-to-understand explanations | Notes |
| Unit 6: Thermochemistry | Key formulas and examples for calorimetry, enthalpy, and Hess’s Law | Notes |
| Unit 7: Equilibrium | Step-by-step logic for pH, buffers, titrations, and Henderson-Hasselbalch; | Notes |
| Unit 8: Acids & Bases | clear explanations of ICE tables, Q vs. K, and Le Chatelier’s Principle | Notes |
| Unit 9: Electrochemistry | Explained: galvanic cells, the Nernst equation, and the relationships between ΔG and K | Notes |
There are nine units in AP Chemistry that cover important topics like atomic structure and electrochemistry. Unit 3 is the most important to focus on because it has the highest exam weight (18–22%).
The table below shows all 9 AP Chemistry units with their official 2025-26 CED titles, exam weights, recommended class periods, and difficulty rating. Use this as your master reference when building your study plan.
| Unit | Unit Name | Exam Weight | Class Periods | Difficulty |
| 1 | Atomic Structure and Properties | 7–9% | 19–20 | Medium |
| 2 | Molecular and Ionic Compound Structure | 7–9% | 22–23 | Medium |
| 3 | Properties of Substances and Mixtures | 18–22% | 14–15 | Medium-Hard |
| 4 | Chemical Reactions | 7–9% | 17–18 | Medium |
| 5 | Kinetics | 7–9% | 15–16 | Hard |
| 6 | Thermochemistry | 7–9% | 10–11 | Medium |
| 7 | Equilibrium | 15–17% | 20–21 | Hard |
| 8 | Acids and Bases | 11–15% | 19–20 | Hard |
| 9 | Electrochemistry | 6–8% | 14–15 | Hard |
Combined Weight of Units 3 + 7 + 8: Up to 54% of the total AP Chemistry exam. Master these three units first.
2024-25 Update: 4 units retitled, Coulomb’s law added to equation sheet, MCQ changed from 5 to 4 answer choices, Topics 7.13 and 7.14 sequenced.
Each unit below includes the key topics tested, must-know formulas, a difficulty note, and one AP-level practice question. Use this section for targeted unit review.
| Formula | What It Calculates |
| n = m / M | Moles from mass and molar mass |
| Avogadro’s Number = 6.022 × 10²³ | Particles per mole |
| Coulomb’s Law: F = k(q₁q₂)/r² | Electrostatic force between charges (NEW on equation sheet 2024-25) |
| Orbital fill order | 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p… |
| Exceptions | Cr = [Ar]3d⁵4s¹; Cu = [Ar]3d¹⁰4s¹ |
Difficulty Note: The hardest things to learn in Unit 1 are how to read PES and how to connect electron configuration to periodic trends. These show up a lot on MCQ.
Practice Question: The configuration of an element is [Ne]3s²3p⁴. Why does it have a lower first ionization energy than [Ne]3s²3p³?
Answer: The 3p⁴ configuration has two electrons that are paired, which makes it easier to remove one electron because they push each other away. The half-filled 3p³ configuration is very stable, which means it takes more energy to ionize.
| Concept | Rule / Formula |
| Formal Charge | FC = valence e⁻ − lone pair e⁻ − ½(bonding e⁻) |
| Electronegativity Difference | 0–0.4 = nonpolar covalent; 0.4–1.7 = polar covalent; >1.7 = ionic |
| Hybridization | 2 domains = sp; 3 domains = sp²; 4 domains = sp³ |
| VSEPR Shapes | Linear, bent, trigonal planar, tetrahedral, trigonal pyramidal, octahedral |
Difficulty Note: You can get to Unit 2 once you know the rules for Lewis structures. People miss formal charge minimization and resonance ranking the most.
Practice Question: Compare the stability of two SO₃ Lewis structures: one with three double bonds and one with two single bonds and one double bond.
Answer: The all-double-bond structure is more stable because the formal charge on S is 0. The formal charge on S in the other structure is +2. Structures with formal charges that are closer to zero are always better.
This is the AP Chemistry unit with the most weight. Don’t underestimate it; gas laws and IMF questions show up on almost every AP Chemistry test.
| Formula | What It Calculates |
| PV = nRT | Ideal Gas Law (R = 0.08206 L·atm/mol·K) |
| P₁V₁/T₁ = P₂V₂/T₂ | Combined Gas Law |
| P_total = P₁ + P₂ + … | Dalton’s Law of Partial Pressures |
| P_A = x_A × P_total | Partial pressure from mole fraction |
| A = εLC | Beer-Lambert Law (absorbance, molar absorptivity, concentration) |
| (P + an²/V²)(V − nb) = nRT | Van der Waals equation for real gases |
| IMF strength order | London dispersion < dipole-dipole < H-bonding < ionic |
Difficulty Note: Unit 3 is the first place where students really run into a big problem. IMF questions often ask you to explain a concept and then compare the boiling points or solubility of two things.
Practice Question: A gas takes up 2.50 L at 25°C and 1.20 atm. At 50°C and 0.800 atm, what is the volume?
Answer: V₂ = V₂ = (P₁V₁T₂)/(P₂T₁) = (1.20 × 2.50 × 323)/(0.800 × 298) = 969/238.4 = 4.07 L. Always change °C to K.
Struggling with gas laws or IMFs?
→ Practice Unit 3 Questions Free at TestPrepKart →
| Formula | What It Calculates |
| % Yield = (actual/theoretical) × 100 | Percent yield of a reaction |
| M₁V₁ = M₂V₂ (1:1 titration) | Molarity from titration |
| OIL RIG | Oxidation Is Loss; Reduction Is Gain |
Priority Flag (Albert.io): The two most important Unit 4 skills on the AP exam are net ionic equations and stoichiometry. The most common mistake that leads to point loss in this unit is losing spectator ions.
Practice Question: Write the net ionic equation for Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq).
Answer: The net ionic equation is Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s). Take out the spectator ions K⁺ and NO₃⁻ because they don’t change on either side.
Unit 5 is one of the hardest parts of the AP Chemistry course. FRQ questions here have many steps, lots of data, and you have to read graphs to figure out the order of the reaction.
| Order | Integrated Rate Law | Linear Plot |
| Zero | [A]t = [A]₀ − kt | [A] vs. time |
| First | ln[A]t = ln[A]₀ − kt | ln[A] vs. time |
| Second | 1/[A]t = 1/[A]₀ + kt | 1/[A] vs. time |
Half-Life (First Order): t½ = 0.693/k
Arrhenius Equation: k = Ae^(−Eₐ/RT) — higher temperature = higher k = faster rate
Practice Question: For a first-order reaction with k = 0.040 s⁻¹ and initial [A] = 0.200 M, find [A] after 15 s.
Answer: ln[A]t = ln[A]t = ln(0.200) − (0.040)(15) = −1.609 − 0.600 = −2.209. [A]t = e^(−2.209) = 0.110 M..
| Formula | What It Calculates |
| q = mcΔT | Heat transferred in calorimetry |
| ΔH°rxn = ΣΔH°f(products) − ΣΔH°f(reactants) | Standard enthalpy of reaction |
| Hess’s Law | ΔH_total = sum of steps; flip sign when reversing; scale when multiplying |
| ΔH (bond enthalpy) | ΣBonds broken − ΣBonds formed |
| Specific heat of water | 4.184 J/g·°C |
Difficulty Note: Unit 6 is one of the easiest units to get into. The most common skills that are tested are Hess’s Law multi-step problems, calorimetry math, and explaining exothermic and endothermic processes.
Practice Question: When you mix 50.0 mL of 1.00 M HCl with 50.0 mL of 1.00 M NaOH, the temperature goes from 21.5°C to 27.3°C. The density is 1.00 g/mL and the specific heat is 4.18 J/g·°C. Determine q for the reaction.
Answer: q_solution = (100.0)(4.18)(5.8) = 2,424 J. q_rxn = −2,424 J = −2.42 kJ. ΔH per mole = −2424/0.0500 = −48,480 J/mol = −48.5 kJ/mol.
Unit 7 is the second most important unit and one of the hardest. There are almost always ICE tables, Q vs. K comparisons, and Le Chatelier’s Principle on both MCQ and FRQ AP Chemistry tests.
| Condition | What Happens |
| Q < K | Reaction proceeds forward (→) toward products |
| Q > K | Reaction proceeds in reverse (←) toward reactants |
| Q = K | System is already at equilibrium — no net change |
| Add reactant | Equilibrium shifts right (forward) |
| Remove product | Equilibrium shifts right (forward) |
| Increase temperature | Shifts toward endothermic direction |
| Increase pressure (gas) | Shifts toward side with fewer moles of gas |
Critical Rule: Never use pure solids or pure liquids in K, Q, Ksp, Ka, or Kb expressions. Only water and gas molecules are included.
Practice Question:At 500K, Kc = 0.020 for N₂(g) + 3H₂(g) ⇌ 2NH₃(g). When [N₂]=0.10 M, [H₂]=0.30 M, and [NH₃]=0.020 M, which way does the reaction go?
Answer: Q = (0.020)²/[(0.10)(0.30)³] = (4×10⁻⁴)/(2.7×10⁻³) = 0.148. The reaction goes in the opposite direction (←) because Q is greater than K (0.148 > 0.020).
The most math-heavy unit in AP Chemistry is Unit 8. Almost every FRQ section has multi-step pH problems, buffer calculations, and titration curves.
| Formula | What It Calculates |
| pH = −log[H⁺] | pH from hydrogen ion concentration |
| pOH = −log[OH⁻] | pOH from hydroxide concentration |
| pH + pOH = 14 (at 25°C) | Relationship between pH and pOH |
| Ka × Kb = Kw = 1.0 × 10⁻¹⁴ | Conjugate acid-base pair relationship |
| pH = pKa + log([A⁻]/[HA]) | Henderson-Hasselbalch (buffer pH) |
| [H⁺] = √(Ka × Ca) | Weak acid approximation (when x << Ca) |
MEMORIZE (NOT on equation sheet): Strong acids: HCl, HBr, HI, HNO₃, H₂SO₄, HClO₄. Strong bases: LiOH, NaOH, KOH, Ca(OH)₂, Ba(OH)₂.
Practice Question: A buffer contains 0.20 mol acetic acid (Ka = 1.8 × 10⁻⁵) and 0.30 mol sodium acetate in 1.00 L. Find the pH.
Answer: pKa = −log(1.8 × 10⁻⁵) = 4.745. pH = 4.745 + log(0.30/0.20) = 4.745 + 0.176 = 4.92.
Unit 9 has the lowest weight for the exam but the hardest concepts. The triangle that connects E°, ΔG°, and K and the Nernst equation are the hardest single relationships in all of AP Chemi
| Formula | What It Calculates |
| E°cell = E°cathode − E°anode | Standard cell potential |
| ΔG° = −nFE°cell | Free energy from cell potential |
| ΔG° = −RT ln K | Free energy from equilibrium constant |
| E = E° − (0.0592/n) log Q | Nernst equation at 25°C |
| n = It / (96485 × n_electrons) | Faraday’s Law (moles deposited) |
| RED CAT, AN OX | Reduction at Cathode; Oxidation at Anode |
Spontaneity Summary: E°cell > 0 → ΔG° < 0 → K > 1 → spontaneous. These three are always linked.
Practice Question: A galvanic cell has E°cell = +0.46 V and n = 2. Calculate ΔG°.
Answer: ΔG° = −nFE° = −(2)(96485)(0.46) = −88,767 J = −88.8 kJ. Spontaneous (ΔG° < 0, E° > 0).
The College Board tests 6 Science Practices in all 9 AP Chemistry units. At least one question on each test checks this. Knowing these things will help you figure out what kind of thought each question needs.
| Science Practice | What It Tests on the Exam |
| 1. Models and Representations | Draw and understand graphs, equations, structural formulas, and diagrams of particles. |
| 2. Question and Method | Find out how to do experiments and plan investigations |
| 3. Representing Data | Make and read graphs, tables, and diagrams with particles |
| 4. Model Analysis | Look at charts, data tables, and experimental results |
| 5. Mathematical Routines | Find the right units to solve multi-step quantitative problems. |
| 6. Argumentation | Back up claims with specific chemical proof and reasoning. |
Expert Tip: The most common skill that students miss on AP Chemistry FRQs is Science Practice 6 (Argumentation). Students know the chemistry, but their reasons are weak. Not just a restatement, every claim needs a specific chemical reason.
| Section | Details |
| Section 1 — Multiple Choice | 60 questions | 90 min | No calculator | 50% of score |
| MCQ Format (2025-26 Update) | 4 answer choices (reduced from 5 in the 2024-25 CED update) |
| Section 2 — Free Response (FRQ) | 7 questions | 105 min | Calculator permitted | 50% of score |
| FRQ Long Questions | 3 questions × 10 points each = 30 points |
| FRQ Short Questions | 4 questions × 4 points each = 16 points |
| Total Exam Time | 195 minutes (3 hours 15 minutes) |
| Provided Materials | Periodic Table + Equations and Constants Sheet (updated 2024-25) |
| Score Range | 1–5 | Score ≥3 earns college credit at 3,300+ U.S. colleges |
| Hybrid Format (2026) | MCQ taken on College Board Bluebook app; FRQ written by hand |
| 2026 Participation | 168,833 students | Mean score: 3.31 | ~58% pass rate (score ≥3) |
Based on Fiveable student ratings, AP educator analysis, and CollegeVine community rankings — here is how all 9 AP Chemistry units rank from hardest to easiest.
| Difficulty Rank | Unit | Why It’s Difficult |
| 1 — Hardest | Unit 8: Acids and Bases | Calculating pH in steps, using buffers, the Henderson-Hasselbalch equation, and titration curves. You need to memorize and do math. |
| 2 | Unit 7: Equilibrium | ICE tables, Q vs. K, and Le Chatelier are both conceptual and quantitative. A lot of pressure comes from a high exam weight. |
| 3 | Unit 9: Electrochemistry | The Nernst equation and the ΔG°-E°-K triangle. The hardest thing to understand in AP Chemistry. |
| 4 | Unit 5: Kinetics | Arrhenius equation, integrated rate laws, and mechanisms. FRQ questions with a lot of graphs. |
| 5 | Unit 3: Properties/Mixtures | IMFs and the laws of gas. Even though this unit is worth a lot on the exam, students don’t give it enough credit. |
| 6 | Unit 6: Thermochemistry | Calorimetry and Hess’s Law. Easier to understand once you know algebra well. |
| 7 | Unit 4: Chemical Reactions | Stoichiometry and net ionic equations. With practice, it’s easy to handle. |
| 8 | Unit 2: Molecular Structure | Hybridization, Lewis structures, and VSEPR. Once you know the rules, it’s easy to guess. |
| 9 — Most Accessible | Unit 1: Atomic Structure | Trends over time and electron configuration. It builds a foundation but is well-organized. |
These are the most frequent point-loss errors across all 9 AP Chemistry units, identified from College Board Chief Reader Reports and AP educator analysis.
| Mistake | Unit(s) Affected | How to Fix It |
| Including solids/liquids in K expressions | Unit 7 | One activity for pure solids and liquids. Never include them in K, Ka, Ksp, or Q. |
| Wrong Le Chatelier shift direction | Unit 7 | To move forward, add a reactant to each stress. To reverse, add product. More T = endothermic direction. |
| Not memorizing strong acids and bases | Unit 8 | Not on the sheet with the equations. Remember these: HCl, HBr, HI, HNO₃, H₂SO₄, HClO₄, NaOH, KOH, and Ba(OH)₂. RED CAT means “reduction at cathode.” AN OX: oxidation at the anode. In galvanic cells, the cathode is positive. |
| Mixing up cathode and anode | Unit 9 | T in Celsius plus 273.15 equals T in Kelvin. Kelvin is always needed for any formula that uses T. |
| Not converting °C to Kelvin | Units 3, 5, 6, 9 | Use chemical terms: name the IMF, say how the reaction works, and name the law that applies. |
| Weak justifications on FRQs | All units | At every step, write down the units. AP readers take points off of final answers that don’t have all the units. |
| Missing units in calculations | All quantitative units | Write units at every step. AP readers deduct points for missing units on final answers. |
Study smarter, not harder. Here is how to allocate your preparation time based on exam weight, difficulty, and AP educator recommendations.
| Priority | Units | Study Action |
| Priority 1 (Start Here) | Units 3, 7, 8 | These 3 units = up to 54% of the exam. Master ICE tables (7), Henderson-Hasselbalch (8), and gas laws + IMFs (3) first. |
| Priority 2 | Units 5, 9 | Hard units with complex FRQ questions. Focus: integrated rate law graphs (5), Nernst equation + ΔG-E-K triangle (9). |
| Priority 3 | Units 1, 2, 4 | Foundation units. Errors here cascade into every later unit. Master electron configuration, Lewis structures, and net ionic equations. |
| Priority 4 | Unit 6 | Most accessible. Focus on q = mcΔT and Hess’s Law. These are reliable FRQ point-earners. |
| Final 4 Weeks | All 9 Units | Complete 2–3 full past AP Chemistry FRQ sets. Use AP Central for official past FRQs with scoring rubrics. Review Chief Reader Reports. |
→ Take a Free AP Chemistry Diagnostic Test at TestPrepKart →
The College Board updated the AP Chemistry Course and Exam Description (CED) for the 2024-25 school year. Here is every change that affects your exam preparation:
| What Changed | Impact on Students |
| 4 units retitled | New official unit names — use 2024-25 CED titles when writing FRQ justifications |
| 6 topics retitled | Topic numbering unchanged; wording updated to reflect current scientific terminology |
| Big Ideas removed from framework | Enduring Understandings are gone — focus on unit-level learning objectives instead |
| Topics 7.13 and 7.14 resequenced | pH and Solubility (7.13) now comes before Free Energy of Dissolution (7.14) for better concept flow |
| Coulomb’s law added to equation sheet | F = k(q₁q₂)/r² now provided during the exam — no need to memorize the formula itself |
| MCQ changed to 4 answer choices | Reduced from 5 choices — improves fairness and reduces time pressure on MCQ section |
| New AP Daily video for Topic 9.6 | New instructional video for Free Energy of Dissolution — use AP Classroom to access it |
| Hybrid exam format (2025) | MCQ delivered on College Board Bluebook app; FRQ still handwritten in paper booklet |
| Resource | Description | Access |
| AP Chemistry Equation Sheet | Key equations and formulas used in AP Chemistry exams | Resource: |
| AP Chemistry Periodic Table | Standard periodic table with atomic data for exam use | Resource: |
| AP Chemistry FRQ | Free-response questions with answers and scoring guidelines | Resource: |
| AP Chemistry Practice Tests | Full-length practice exams with MCQs and FRQs | Resource: |
| AP Chemistry Formula Sheet | Complete formula sheet for quick revision | Resource: |
| AP Chemistry Reference Sheet | Official reference sheet with constants and equations | Resource: |
| AP Chemistry Worksheets | Topic-wise worksheets for practice and revision | Resource: |
How many units are in AP Chemistry?
The College Board’s AP Chemistry Course and Exam Description (CED) says that AP Chemistry has nine units. They go from Unit 1: Atomic Structure to Unit 9: Electrochemistry. The 2024-25 CED is the most up-to-date official source for unit names and exam weights.
Which AP Chemistry unit has the highest exam weight?
Unit 3, which covers the properties of substances and mixtures, is the most important part of the AP Chemistry exam, making up 18–22% of the total. It talks about intermolecular forces, gas laws, and the Beer-Lambert Law. Unit 7 (Equilibrium) comes in second with 15–17% of the total, and Unit 8 (Acids and Bases) comes in third with 11–15%..
What is the hardest AP Chemistry unit?
Most AP Chemistry students say that Unit 8 (Acids and Bases) is the hardest because it has the most calculations. Unit 7 (Equilibrium) and Unit 9 (Electrochemistry) come right after that. A lot of people say that the Nernst equation in Unit 9, which connects E°cell, ΔG°, and K, is the hardest thing to understand in all of AP Chemistry.
What changed in AP Chemistry for 2024-25?
The 2024–25 CED update changed the names of 4 units, 6 topics, and 6 topics. It also took out Big Ideas, changed the order of Topics 7.13–7.14, added Coulomb’s law to the equation sheet, cut the number of answer choices for the MCQ from 5 to 4, and added a new AP Daily video for Topic 9.6 (Free Energy of Dissolution).
Is a calculator allowed on the AP Chemistry exam?
You can only use a calculator on the free-response (FRQ) part of the AP Chemistry test. You can’t use a calculator on the 60-question multiple-choice part. For MCQ, practice mental math and estimation, especially pH approximations and stoichiometry.
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