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Unit 1: Atomic Structure and Properties · Practice test

AP Chemistry Unit 1 Practice Test

Fifteen exam-style multiple-choice questions covering every Unit 1 topic, with instant scoring and an explanation for each answer.

Unit 1, Atomic Structure and Properties, is 7–9% of the multiple-choice section. These questions follow the AP format: four choices, one correct answer, a calculator and periodic table 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 1 review if you need to.

1 How many atoms are in 2.00 mol of water, H2O?
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Answer: B. 2.00 mol × 6.022 × 1023 = 1.20 × 1024 molecules, and each molecule has 3 atoms, so 3.61 × 1024 atoms. Choice A counts molecules, not atoms.

2 What is the mass of 0.250 mol of calcium nitrate, Ca(NO3)2?
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Answer: B. The molar mass is 40.08 + 2(14.01) + 6(16.00) = 164.10 g/mol, and 0.250 mol × 164.10 g/mol = 41.0 g. Choice A forgets that the subscript 2 doubles the whole nitrate group.

3 A mass spectrum of an element shows two peaks: mass 63 at 69.2% abundance and mass 65 at 30.8%. Which element is it?
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Answer: B. The weighted average is 63(0.692) + 65(0.308) = 63.6, which matches copper (63.55). The average must lie closer to the more abundant isotope, so it cannot be zinc (65.38).

4 In the mass spectrum of a pure element, what does the height of each peak represent?
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Answer: B. Peak position on the horizontal axis gives the mass; peak height gives how common that isotope is. All isotopes of an element have the same number of protons.

5 A hydrocarbon is 85.6% carbon and 14.4% hydrogen by mass. What is its empirical formula?
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Answer: B. In 100 g: 85.6 ÷ 12.01 = 7.13 mol C and 14.4 ÷ 1.008 = 14.3 mol H, a 1 : 2.00 ratio, so CH2.

6 Pure water from a glacier and pure water made in a lab are both 11.2% hydrogen by mass. Which idea does this illustrate?
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Answer: B. A pure compound always has the same ratio of element masses, whatever its source. That is the law of definite proportions, which the AP course ties to empirical formulas.

7 A 2.00 g mixture of NaCl and sand (SiO2) contains 0.355 g of chlorine. What is the mass percent of NaCl in the mixture?
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Answer: B. 0.355 g Cl ÷ 35.45 g/mol = 0.0100 mol Cl = 0.0100 mol NaCl, which is 0.585 g NaCl. 0.585 ÷ 2.00 = 29.3%. Choice A is the percent of chlorine, not of NaCl.

8 Which is the ground-state electron configuration of a sulfur atom?
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Answer: A. Sulfur has 16 electrons: 2 + 2 + 6 + 2 + 4. Choice B is S2−, and choice C is an excited state because 3s is not full.

9 Which is the electron configuration of the Fe3+ ion?
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Answer: B. Fe is [Ar] 4s2 3d6. Cations lose electrons from the highest shell first, so both 4s electrons go, then one 3d electron: [Ar] 3d5. Choice A removes 3d electrons first.

10 The photoelectron spectrum of magnesium has how many peaks, and what are their relative heights from highest to lowest binding energy?
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Answer: B. Mg is 1s2 2s2 2p6 3s2: one peak per subshell, with heights proportional to the electrons in each, listed from 1s (highest binding energy) to 3s.

11 In any photoelectron spectrum, the peak with the highest binding energy comes from electrons in which subshell?
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Answer: B. 1s electrons are closest to the nucleus and least shielded, so their Coulombic attraction is strongest and the most energy is needed to remove them.

12 Which atom has the largest atomic radius?
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Answer: A. All four have valence electrons in the third shell. Moving right adds protons, so the nucleus pulls those electrons closer. Sodium, with the fewest protons, is largest.

13 A period 3 element has successive ionization energies of 738, 1451, 7733, and 10,540 kJ/mol. Which element is it?
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Answer: B. The large jump comes after the second electron, so the third must come from a filled inner shell. The element has 2 valence electrons: magnesium.

14 Which has the greater first ionization energy, F or Cl, and why?
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Answer: A. F’s valence electrons are in the second shell and Cl’s in the third. The extra protons in Cl are offset by more shielding, so the closer electrons of F feel the stronger attraction.

15 What is the formula of the ionic compound formed by aluminum and oxygen?
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Answer: B. Aluminum loses its 3 valence electrons to form Al3+; oxygen gains 2 to form O2−. Two Al3+ (+6) balance three O2− (−6).

What the Unit 1 Practice Test Covers

CED topicQuestionsReview guide
1.1 Moles and Molar Mass1, 2Mole conversions
1.2 Mass Spectra of Elements3, 4Average atomic mass
1.3 Elemental Composition of Pure Substances5, 6Empirical and molecular formulas
1.4 Composition of Mixtures7Pure substances and mixtures
1.5 Atomic Structure and Electron Configuration8, 9Electron configurations
1.6 Photoelectron Spectroscopy10, 11Photoelectron spectroscopy
1.7 Periodic Trends12, 13, 14Periodic trends
1.8 Valence Electrons and Ionic Compounds15Valence electrons

A score of 12 or more out of 15 suggests Unit 1 is solid. Below that, open the guides for the topics listed in your results, then try the Unit 1 FRQ practice, which asks you to explain the same ideas in writing. The electron configuration calculator and molar mass calculator are useful for checking your work on questions like 2, 8, and 9. Your Unit 1 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 1 practice test?

Fifteen multiple-choice questions, at least one for each of the eight Unit 1 topics in the course description. Each has four choices and a written explanation.

How much of the AP Chemistry exam is Unit 1?

Unit 1, Atomic Structure and Properties, makes up 7 to 9 percent of the multiple-choice section, about 4 to 5 of the 60 questions.

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.