AP Chemistry free-response questions: calculations and explanations

The free-response section

Source: AP Central, AP Chemistry exam page, checked 7 October 2026. 1 hour 45 minutes for all seven; 50% of the exam score. Reference materials are provided in Bluebook and on paper.

What the questions ask for

Long questions usually follow one scenario through several parts: a reaction, a lab procedure, a set of data. One part asks for a calculation, the next for a particle-level explanation, the next for a prediction if a condition changes. Short questions are tighter, often a single calculation and an explanation.

Calculations earn points for a correct setup and answer with units. Show the relationship you are using, substitute with units, and give a sensible number of significant figures. Explanations earn points for naming the relevant particles and interactions: ions, dipoles, hydrogen bonds, London dispersion forces, collisions with sufficient energy and correct orientation. 'Because it is stronger' without saying what is stronger does not earn the point.

Worked example 1: titration

A 25.00 mL sample of hydrochloric acid is titrated with 0.200 M sodium hydroxide. The equivalence point is reached after 18.40 mL of NaOH. (a) Write the net ionic equation. (b) Calculate the concentration of the acid.

1. (a) H+(aq) + OH-(aq) → H2O(l). Sodium and chloride ions are spectators, so they are left out.
2. (b) Moles of OH- = 0.200 mol/L × 0.01840 L = 0.003680 mol.
3. The ratio is 1:1, so moles of H+ = 0.003680 mol.
4. Concentration of HCl = 0.003680 mol ÷ 0.02500 L = 0.147 M (three significant figures, matching 0.200 M).

Worked example 2: pH of a weak acid

Calculate the pH of 0.10 M HA, a weak acid with Ka = 1.8 × 10^-5. Justify any approximation.

1. HA ⇌ H+ + A-. At equilibrium [H+] = [A-] = x and [HA] = 0.10 - x.
2. Ka = x^2/(0.10 - x). Because Ka is small, assume x is much smaller than 0.10, so x^2 = 1.8 × 10^-6.
3. x = 1.34 × 10^-3 M, so pH = -log(1.34 × 10^-3) = 2.87.
4. Check: 1.34 × 10^-3 is about 1.3% of 0.10, well under 5%, so the approximation is justified. Saying so in words earns the reasoning point.

Worked example 3: enthalpy from calorimetry

When 0.0250 mol of acid is neutralized in a coffee-cup calorimeter containing 50.0 g of solution, the temperature rises by 6.5 °C. Take the specific heat capacity as 4.18 J/(g·°C). Calculate the enthalpy change per mole.

1. Heat absorbed by the solution: q = m × c × ΔT = 50.0 × 4.18 × 6.5 = 1,358.5 J, about 1.36 kJ.
2. The reaction released this heat, so it is exothermic and ΔH is negative.
3. ΔH = -1.3585 kJ ÷ 0.0250 mol = -54.34 kJ/mol. ΔT is given to two significant figures, so report -54 kJ/mol.
4. Common slip: forgetting the negative sign, or dividing by the mass instead of the moles.

Common mistakes

• Explaining boiling points with 'stronger bonds' when the question is about intermolecular forces, not covalent bonds within molecules.
• Including spectator ions in a net ionic equation.
• Missing units or using milliliters in a molarity calculation without converting to liters.
• Wrong sign on ΔH, or confusing heat released by the reaction with heat absorbed by the water.
• Using an approximation in an equilibrium calculation without checking it.
• Answering 'it shifts right' for Le Chatelier questions without saying why in terms of Q and K.

How one-to-one lessons help with AP Chemistry FRQs

AP Chemistry rewards students who can move between numbers and particles. In lessons the tutor sets one long question under time, then works through each part with the student on the shared whiteboard: the calculation setup, the units and significant figures, and the particle-level sentence that would earn the explanation point. Students build a short bank of reasoning templates for the recurring explanations: intermolecular forces, Le Chatelier, buffers, kinetics and electrochemistry.

College Board's released free-response questions and scoring guidelines on AP Central are the core practice material. Unit 3 (properties of substances and mixtures) and acids and bases carry the most multiple-choice weight and also feature often in free response, so they usually get extra time.

Self-check

• Can you explain why ethanol has a higher boiling point than propane using intermolecular forces?
• Can you write a net ionic equation for a precipitation reaction?
• Can you justify the 5% approximation in a weak-acid calculation?
• Can you use Q versus K to predict the direction of a reaction?
• Can you calculate ΔH from calorimetry data with the correct sign?