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Unit 3: Properties of Substances and Mixtures · Practice test

AP Chemistry Unit 3 Practice Test

Fifteen exam-style multiple-choice questions covering all thirteen Unit 3 topics, with instant scoring and an explanation for each answer.

Unit 3, Properties of Substances and Mixtures, is the most heavily weighted unit on the exam at 18–22% of the multiple-choice section, about 11 to 13 of the 60 questions. These questions follow the AP format: four choices, one correct answer, and a calculator, periodic table, and reference sheet allowed. Give yourself about 22 minutes, the exam’s pace of 90 seconds per question, then press Check My Answers. Where a wrong choice is especially tempting, its explanation says why. Review the content first in the Unit 3 review if you need to.

1 Which substance has the highest boiling point?
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Answer: C. The molar masses are similar, so dispersion forces are comparable. Only ethanol has an H bonded to O, so only ethanol molecules form hydrogen bonds with each other. Choice B has an oxygen atom but every hydrogen is bonded to carbon, so it has dipole-dipole forces but no hydrogen bonding.

2 At room temperature, I2 is a solid and Cl2 is a gas. Which statement best explains this difference?
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Answer: A. Both molecules are nonpolar, so dispersion forces are the only intermolecular forces, and they grow with the number of electrons. Choice B is a common trap: changing phase does not break the covalent bond, and the I–I bond is actually weaker than the Cl–Cl bond.

3 A white solid melts at 770°C. It does not conduct electricity as a solid, but it conducts when melted or dissolved in water. What type of solid is it most likely to be?
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Answer: D. A high melting point plus conductivity only when the ions are free to move is the signature of an ionic solid. A metallic solid conducts as a solid, a molecular solid melts low and does not conduct, and a covalent network solid such as quartz does not conduct even when melted.

4 Water vapor condenses to liquid water at a constant 100°C. Which statement describes what happens at the particle level?
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Answer: B. Condensation is exothermic: attractions form between molecules as they come close. The temperature is constant, so the average kinetic energy does not change (Choice A), and the molecules in a liquid still move past one another.

5 What is the pressure of 0.500 mol of an ideal gas held at 300. K in a 10.0 L container? (R = 0.08206 L·atm/(mol·K))
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Answer: D. P = nRT/V = (0.500)(0.08206)(300.) ÷ 10.0 = 1.23 atm. Choice B uses 27°C in place of 300. K; the ideal gas law always needs kelvins. Choice A is V/(nRT), the expression upside down.

6 A diatomic gas has a density of 1.31 g/L at 298 K and 1.00 atm. Which gas is it?
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Answer: A. Rearranging PV = nRT with n = mass/M gives M = dRT/P = (1.31)(0.08206)(298) ÷ 1.00 = 32.0 g/mol, which matches O2 (32.00 g/mol). N2 is 28.02 g/mol and Cl2 is 70.90 g/mol.

7 Samples of helium and argon are at the same temperature. Which statement is correct?
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Answer: C. Temperature sets the average kinetic energy, ½mv2, regardless of the gas. With the same kinetic energy, the lighter helium atoms must have the higher average speed.

8 Under which conditions does a real gas such as NH3 behave least like an ideal gas?
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Answer: B. At high pressure the molecules are crowded, so their own volume matters, and at low temperature they move slowly enough for intermolecular attractions to pull them together. Both break the assumptions of kinetic molecular theory. Choice A describes the conditions where gases are most ideal.

9 A student dissolves 5.85 g of NaCl (58.44 g/mol) in water to make 250. mL of solution. What is the molarity of the solution?
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Answer: A. 5.85 g ÷ 58.44 g/mol = 0.100 mol, and 0.100 mol ÷ 0.250 L = 0.400 M. Choice B stops at moles, and Choices C and D divide grams by volume without converting to moles.

10 A particle diagram shows CaCl2 dissolved in water. Which feature must a correct diagram include?
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Answer: D. Each formula unit releases one Ca2+ and two Cl−. The partially negative oxygen end of water faces the cation and the partially positive hydrogen end faces the anion. Choice A has the ratio right but the water reversed.

11 In paper chromatography, the solvent front moves 8.0 cm. Dye X moves 6.0 cm and dye Y moves 2.4 cm. What is the Rf of dye Y, and which dye is more strongly attracted to the paper?
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Answer: B. Rf = distance moved by the spot ÷ distance moved by the solvent = 2.4 ÷ 8.0 = 0.30. The dye that travels the shorter distance spends more time attached to the stationary phase, the paper, so dye Y is more strongly attracted to it. An Rf can never exceed 1, which rules out Choice D.

12 Which substance is most soluble in hexane, C6H14?
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Answer: C. Hexane is nonpolar and attracts other particles only through dispersion forces. Nonpolar I2 interacts with hexane the same way, so it dissolves. The ionic solids and hydrogen-bonding methanol would have to give up much stronger attractions that hexane cannot replace.

13 Which type of electromagnetic radiation is associated with transitions in the vibrational energy levels of molecules?
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Answer: D. Infrared radiation makes bonds stretch and bend. Microwave radiation is associated with rotational transitions, and visible and ultraviolet radiation with electronic transitions.

14 What is the energy of one photon of violet light with a wavelength of 400. nm? (h = 6.626 × 10−34 J·s, c = 2.998 × 108 m/s)
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Answer: A. E = hc/λ = (6.626 × 10−34)(2.998 × 108) ÷ (4.00 × 10−7 m) = 4.97 × 10−19 J. Choice B leaves the wavelength in nanometers, and Choice C is the frequency in s−1, not an energy.

15 A solution in a 1.00 cm cuvette has an absorbance of 0.450 at a wavelength where the molar absorptivity is 150. M−1cm−1. What is its concentration?
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Answer: B. A = εbc, so c = A/(εb) = 0.450 ÷ (150. × 1.00) = 3.00 × 10−3 M. Choice A multiplies instead of dividing, and Choice C divides the wrong way.

What the Unit 3 Practice Test Covers

CED topicQuestionsReview guide
3.1 Intermolecular and Interparticle Forces1, 2Intermolecular forces
3.2 Properties of Solids3Types of solids
3.3 Solids, Liquids, and Gases4Heating curves
3.4 Ideal Gas Law5, 6Ideal gas law
3.5 Kinetic Molecular Theory7Kinetic molecular theory
3.6 Deviation From Ideal Gas Law8Real gases
3.7 Solutions and Mixtures9Molarity
3.8 Representations of Solutions10Solutions
3.9 Separation of Solutions and Mixtures11Chromatography
3.10 Solubility12Solubility
3.11 Spectroscopy and the Electromagnetic Spectrum13Spectroscopy
3.12 Properties of Photons14Photon energy
3.13 Beer-Lambert Law15Beer-Lambert law

A score of 12 or more out of 15 suggests Unit 3 is solid. Below that, open the guides for the topics listed in your results. Question 2 is the classic trap of the unit, confusing the forces between molecules with the bonds inside them; the difference is laid out in intermolecular vs intramolecular forces. The gas law calculator, molarity calculator, and wavelength calculator check your arithmetic on questions 5, 6, 9, and 14. Your Unit 3 result is one input; the AP Chem Score Calculator shows how a full practice exam turns into a 1–5 estimate.

Frequently Asked Questions

How many questions are in the AP Chemistry Unit 3 practice test?

Fifteen multiple-choice questions, at least one for each of the thirteen Unit 3 topics in the course description, with two each for intermolecular forces and the ideal gas law. Each has four choices and a written explanation.

How much of the AP Chemistry exam is Unit 3?

Unit 3, Properties of Substances and Mixtures, makes up 18 to 22 percent of the multiple-choice section, about 11 to 13 of the 60 questions. It is the most heavily weighted unit on the exam.

Can I use a calculator on the Unit 3 questions?

Yes. A calculator is allowed on both sections of the AP Chemistry exam, so gas law, molarity, photon, and Beer-Lambert questions can be worked with one, along with the periodic table and reference sheet.

Are these real AP Chemistry questions?

No. They are original questions written in the AP style for this site and checked against the course description. Released College Board free-response questions are worked separately on this site.

These are original practice questions written for this site, not released College Board questions. AP® is a trademark registered by the College Board, which is not affiliated with and does not endorse this site.