AP Chemistry · Handsworth Secondary 2026–27
Unit 3 · Study Guide
Dr. Ras Mulinta
Properties of Substances & Mixtures
Exam-focused review
A one-page map of what the Unit 3 test (and the AP exam) expects. This is the heaviest-weighted unit on the AP exam, 18–22%, so the IMF reasoning and the gas/solution math here pay off everywhere. This is a checklist, not a re-teach, if a line doesn't click, go back to that section of the notes package. A periodic table, the gas constant, c, and h are provided on the exam.
Must be able to do
- 3.1 Rank London dispersion < dipole–dipole < hydrogen bonding from a molecule's structure; name the actual force (never just "strong/weak") and tie it to boiling point / vapour pressure.
- 3.2–3.3 Classify ionic / covalent-network / molecular / metallic solids and justify mp, hardness, and conductivity; draw solid/liquid/gas particle models.
- 3.4 Use PV = nRT (in kelvin); apply partial pressures P_A = X_A·P_total, P_total = ΣP_i.
- 3.5–3.6 Use KMT (KE = ½mv², T ∝ avg KE); explain real-gas deviation, worst at high P (particle volume) and low T (attractions).
- 3.7–3.8 Compute molarity M = n/L and dilutions M₁V₁ = M₂V₂; read particulate pictures for relative concentration.
- 3.9–3.10 Explain filtration vs. distillation vs. chromatography by intermolecular interactions; apply "like dissolves like."
- 3.11–3.12 Match EM region → transition (microwave→rotation, IR→vibration, UV-Vis→electronic); use c = λν and E = hν.
- 3.13 Apply Beer–Lambert A = εbc; with b, λ fixed, A ∝ c (calibration line), this is the brass/Cu lab.
The big idea that ties it together
Intermolecular forces set physical properties. The whole unit is one question asked many ways: how strongly do these particles attract each other? Stronger IMFs → higher boiling/melting points, lower vapour pressure, tighter packing, and bigger deviations from the ideal gas law. The same "like attracts like" logic explains solubility, chromatography, and distillation. The gas-law and Beer–Lambert math just quantify what the particles are doing.
Traps that cost marks
°C not converted to K. Every gas-law calculation (PV = nRT, P₁/T₁ = P₂/T₂) needs kelvin. Fix: write K = °C + 273.15 as the first line, every time.
Wrong IMF / bad ranking. "Stronger/weaker" is not an answer, and hydrogen bonding is easy to over- or under-call. Fix: name the actual force; H-bonding needs H directly on N, O, or F, otherwise it's dipole–dipole or (if nonpolar) dispersion, and for big molecules dispersion can win.
Total pressure used for partial pressure. Dalton needs the mole fraction. Fix: P_A = X_A·P_total; for gas collected over water, subtract the water vapour pressure first.
Assuming ideal behaviour where it breaks. Real gases deviate at high P (particle volume matters) and low T (attractions matter). Fix: flag deviation near those conditions; ideal is best at low P, high T.
Beer–Lambert slip-ups. A is unitless and directly proportional to both c and path length b. Fix: A = εbcdouble c (or double b) doubles A; keep readings in the linear range (A ≲ 1).
Molarity uses solution, not solvent. M = moles per litre of final solution, and it's per litre not millilitre. Fix: M = n/L_solution; convert mL → L before dividing.
Don't waste time on (excluded by the CED)
Phase diagrams (interpreting P–T diagrams) · colligative properties · calculations of molality, percent-by-mass, and percent-by-volume. Know molarity and dilution cold; skip these.
Quick self-check (answer in your head, then verify)
- Which boils higher, HF or HCl? Name the deciding force.
- A flask holds 1.0 mol N₂ and 3.0 mol O₂ at 4.0 atm total. What is the partial pressure of O₂?
- Molarity of 0.0500 mol solute dissolved to 250.0 mL of solution?
- You dilute 10.0 mL of 6.0 M HCl to a final 60.0 mL. New molarity?
- Moles of ideal gas in 3.00 L at 1.00 atm and 300. K? (R = 0.08206)
- Beer–Lambert: c = 4.00×10⁻³ M, b = 1.00 cm, ε = 100. L·mol⁻¹·cm⁻¹. Find A.
- Under which conditions does a real gas behave most ideally, low or high P, low or high T?
Check yourself: 1) HF: hydrogen bonding (H–F) beats HCl's dipole–dipole. 2) X(O₂) = 3.0/4.0 = 0.75 → 3.0 atm. 3) 0.0500/0.2500 = 0.200 M. 4) (6.0)(10.0)/(60.0) = 1.0 M. 5) n = PV/RT = (1.00)(3.00)/(0.08206·300.) = 0.122 mol. 6) A = εbc = (100.)(1.00)(4.00×10⁻³) = 0.400. 7) low P and high T (small particle volume, weak attractions).