I85PREP
⌘ K
CLEP Chemistry study materials
CLEP Chemistry study resources

CLEP Chemistry practice questions

One question at a time. Build your CLEP Chemistry understanding.

First 50 of 200 questions. Each one lists the choices, the correct answer, and an explanation. The checker at the top of this page saves progress in this browser when it is available.

Continue in the practice tool above to load more questions, 50 at a time.

Question 1Structure of Matter

How many protons does an atom of chlorine-37 contain?

  1. 37
  2. 17 Correct answer.
  3. 20
  4. 18

Explanation

The atomic number of chlorine is 17, and the atomic number equals the number of protons. The mass number 37 is the sum of protons and neutrons, so it does not change the proton count.

Question 2Structure of Matter

Which subatomic particle has essentially no mass compared with the others?

  1. Proton
  2. Electron Correct answer.
  3. Neutron
  4. Alpha particle

Explanation

The electron mass is about 1/1836 of a proton or neutron mass, so it is negligible on the atomic scale. Protons and neutrons each have roughly one atomic mass unit.

Question 3Structure of Matter

Which statement describes isotopes of an element?

  1. Atoms with the same number of electrons but different protons
  2. Atoms with the same number of neutrons but different protons
  3. Atoms with equal numbers of protons and neutrons
  4. Atoms with the same number of protons but different neutrons Correct answer.

Explanation

Isotopes are atoms of the same element, so they share the atomic number (proton count) but differ in neutron count and therefore mass number. Changing the proton count would change the element.

Question 4Structure of Matter

What is the electron configuration of a neutral sulfur atom (Z = 16) in its ground state?

  1. 1s2 2s2 2p6 3s2 3p4 Correct answer.
  2. 1s2 2s2 2p6 3s2 3p6
  3. 1s2 2s2 2p6 3s2 3p5
  4. 1s2 2s2 2p6 3s2 3p2

Explanation

Sixteen electrons fill orbitals in order of increasing energy: 1s (2), 2s (2), 2p (6), 3s (2), leaving 4 for 3p. The total of 2+2+6+2+4 equals 16.

Question 5Structure of Matter

In the modern periodic table, elements within a group generally share which characteristic?

  1. The same atomic mass
  2. The same number of neutrons
  3. The same number of valence electrons Correct answer.
  4. The same number of electron shells

Explanation

A group is a vertical column, and its members have the same valence electron count, which gives them similar chemical behavior. Periods, not groups, correspond to the number of occupied shells.

Question 6Structure of Matter

An ionic bond is best described as the electrostatic attraction between which species?

  1. Oppositely charged ions Correct answer.
  2. Delocalized electrons and metal cations
  3. Two partially charged molecular dipoles
  4. Shared electron pairs between two nonmetals

Explanation

Ionic bonding results from electron transfer, producing cations and anions that attract electrostatically. Sharing electrons describes covalent bonds, and delocalized electrons in a metal describe metallic bonding.

Question 7Structure of Matter

Which quantity is conserved in a balanced chemical equation?

  1. The number of moles of gas
  2. The number of molecules
  3. The total number of atoms of each element Correct answer.
  4. The total volume of gas

Explanation

Balancing reflects the law of conservation of mass, which requires the same number of atoms of each element on both sides. Molecule counts and gas volumes can change during reaction.

Question 8Structure of Matter

How many significant figures are in the measurement 0.004050 g?

  1. 6
  2. 3
  3. 7
  4. 4 Correct answer.

Explanation

Leading zeros are not significant; the digits 4, 0, 5, and the trailing 0 after the decimal are significant, giving four. Trailing zeros after the decimal point count because they indicate precision.

Question 9Structure of Matter

Which element has the largest atomic radius?

  1. Chlorine
  2. Magnesium
  3. Sodium Correct answer.
  4. Argon

Explanation

Across a period, increasing nuclear charge pulls electrons closer and shrinks the radius, so radius decreases from left to right. Sodium lies furthest left of the listed elements and is therefore largest.

Question 10Structure of Matter

A neutral atom has the electron configuration 1s2 2s2 2p6 3s2 3p6 4s1. Which element is it?

  1. Sodium
  2. Argon
  3. Calcium
  4. Potassium Correct answer.

Explanation

The configuration totals 19 electrons, matching the atomic number of potassium. The single 4s electron is characteristic of the first element of period 4.

Question 11Structure of Matter

Which pair of elements would most likely form an ionic compound when combined?

  1. A metal and a nonmetal such as potassium and chlorine Correct answer.
  2. Two nonmetals such as carbon and oxygen
  3. Two noble gases such as helium and neon
  4. Two metals such as sodium and magnesium

Explanation

Ionic bonding typically arises when a low-ionization metal transfers electrons to a high-electron-affinity nonmetal. Combinations of two nonmetals tend to share electrons covalently.

Question 12Structure of Matter

Which molecule has a trigonal pyramidal molecular geometry?

  1. CH4
  2. NH3 Correct answer.
  3. CO2
  4. BF3

Explanation

Nitrogen in ammonia has three bonding pairs and one lone pair, so the electron geometry is tetrahedral and the molecular geometry is trigonal pyramidal. Lone pairs are not counted as atoms when naming the shape.

Question 13Structure of Matter

Which type of intermolecular force is primarily responsible for the relatively high boiling point of water?

  1. Ion–dipole forces
  2. Hydrogen bonding Correct answer.
  3. Dipole–dipole forces
  4. London dispersion forces

Explanation

Water molecules can form hydrogen bonds because hydrogen is bonded to a highly electronegative oxygen atom with lone pairs. These relatively strong attractions require more energy to overcome.

Question 14Structure of Matter

Which sample contains the greatest number of atoms?

  1. 1.0 mol of CH4 Correct answer.
  2. 1.0 mol of H2O
  3. 1.0 mol of Ne
  4. 1.0 mol of O2

Explanation

One mole of methane contains one carbon plus four hydrogen atoms, for a total of five moles of atoms. The other samples contain fewer atoms per formula unit.

Question 15Structure of Matter

What is the mass percent of oxygen in water (H2O)? Use H = 1.0 g/mol and O = 16.0 g/mol.

  1. 11.1%
  2. 16.0%
  3. 50.0%
  4. 88.9% Correct answer.

Explanation

The molar mass of water is 18.0 g/mol, of which 16.0 g is oxygen. Dividing 16.0 by 18.0 and multiplying by 100 gives about 88.9%.

Question 16Structure of Matter

Which ion has the same electron configuration as a neon atom?

  1. Ca2+
  2. Na+ Correct answer.
  3. Cl−
  4. S2−

Explanation

Sodium has 11 electrons, so losing one gives 10 electrons, matching neon's configuration. Ions isoelectronic with neon must also have 10 electrons.

Question 17Structure of Matter

How many protons, neutrons, and electrons are in the ion 56Fe3+ (Z = 26)?

  1. 29 protons, 27 neutrons, 29 electrons
  2. 23 protons, 33 neutrons, 26 electrons
  3. 26 protons, 30 neutrons, 29 electrons
  4. 26 protons, 30 neutrons, 23 electrons Correct answer.

Explanation

Iron has 26 protons; the mass number 56 minus 26 gives 30 neutrons. A 3+ charge means three electrons were lost, leaving 23 electrons.

Question 18Structure of Matter

Which list orders the elements by increasing electronegativity?

  1. K < Na < Li < F
  2. Na < Si < P < Cl Correct answer.
  3. Cl < P < Si < Na
  4. F < Cl < Br < I

Explanation

Electronegativity increases left to right across a period and increases up a group. Sodium, silicon, phosphorus, and chlorine run left to right in period 3, so this order is increasing.

Question 19Structure of Matter

Which compound contains both ionic and covalent bonding?

  1. NaCl
  2. CH4
  3. Cl2
  4. NaOH Correct answer.

Explanation

Sodium hydroxide contains an ionic bond between Na+ and OH− and a covalent bond between oxygen and hydrogen. Compounds with only one type of bonding do not meet the criterion.

Question 20Structure of Matter

Which molecule is nonpolar even though it contains polar bonds?

  1. H2O
  2. CCl4 Correct answer.
  3. NH3
  4. HCl

Explanation

The four C–Cl bond dipoles in carbon tetrachloride point symmetrically and cancel, making the molecule nonpolar overall. Polar bonds do not guarantee a polar molecule.

Question 21Structure of Matter

What is the empirical formula of a compound that is 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen by mass?

  1. C2H6O
  2. C2H4O2
  3. CH2O Correct answer.
  4. CHO

Explanation

Dividing each percentage by its atomic mass gives 3.33 mol C, 6.7 mol H, and 3.33 mol O; dividing by the smallest gives the ratio 1:2:1. The empirical formula is CH2O.

Question 22Structure of Matter

Which statement correctly describes the trend in first ionization energy across period 3 from sodium to argon?

  1. It stays constant because valence shells do not change
  2. It decreases steadily because atoms get larger
  3. It generally increases because effective nuclear charge rises Correct answer.
  4. It generally decreases because shielding increases

Explanation

Across a period, protons are added while electrons enter the same shell, so effective nuclear charge increases and electrons are held more tightly. This raises ionization energy overall.

Question 23Structure of Matter

A 2.50 g sample of a hydrated salt is heated until all water of hydration is driven off, leaving 1.60 g of anhydrous salt. What is the mass percent of water in the hydrate?

  1. 64.0%
  2. 25.0%
  3. 36.0% Correct answer.
  4. 56.0%

Explanation

The mass of water lost is 2.50 − 1.60 = 0.90 g. Dividing 0.90 g by the original 2.50 g and multiplying by 100 gives 36.0%.

Question 24Structure of Matter

Which set of quantum numbers (n, l, ml, ms) is allowed for an electron in an atom?

  1. 3, 2, −2, +1/2 Correct answer.
  2. 2, 2, 0, +1/2
  3. 1, 0, 1, −1/2
  4. 3, −1, 0, +1/2

Explanation

The rules require l to range from 0 to n−1 and ml to range from −l to +l, with ms equal to ±1/2. For n = 3, l = 2 is allowed and ml = −2 lies within the permitted range.

Question 25Structure of Matter

Two samples of the same compound are decomposed, one yielding 4.0 g of carbon with 10.7 g of oxygen and the other yielding 6.0 g of carbon with 16.0 g of oxygen. These data best illustrate which law?

  1. Gay-Lussac's law of combining volumes
  2. The law of conservation of energy
  3. The law of definite proportions Correct answer.
  4. The law of multiple proportions

Explanation

In both samples the oxygen-to-carbon mass ratio is the same (about 2.67 to 1), so the compound has a fixed composition by mass. This is the law of definite proportions.

Question 26Structure of Matter

Which subatomic particle determines the identity of an element?

  1. Photon
  2. Electron
  3. Neutron
  4. Proton Correct answer.

Explanation

The number of protons (atomic number) uniquely identifies an element. Neutrons affect the isotope, electrons affect charge and bonding, and photons are not part of the atom.

Question 27Structure of Matter

What is the maximum number of electrons that can occupy a single p orbital?

  1. 2 Correct answer.
  2. 6
  3. 10
  4. 14

Explanation

Each orbital can hold at most two electrons, with opposite spins, according to the Pauli exclusion principle. A p subshell has three orbitals and holds six electrons, but a single p orbital holds two.

Question 28Structure of Matter

Which of the following is an isotope of hydrogen?

  1. Helium-4
  2. Carbon-12
  3. Lithium-7
  4. Deuterium Correct answer.

Explanation

Isotopes of an element have the same number of protons but different numbers of neutrons. Deuterium has one proton and one neutron, making it an isotope of hydrogen.

Question 29Structure of Matter

Which statement best describes the arrangement of particles in a solid?

  1. Particles are closely packed but can slide past one another.
  2. Particles are closely packed and vibrate about fixed positions. Correct answer.
  3. Particles are ionized and move independently.
  4. Particles are far apart and move freely in all directions.

Explanation

In a solid, particles are held in fixed positions by intermolecular or ionic forces and can only vibrate. This gives solids a definite shape and volume.

Question 30Structure of Matter

Which of the following elements is a metalloid?

  1. Silicon Correct answer.
  2. Sulfur
  3. Sodium
  4. Argon

Explanation

Metalloids have properties intermediate between metals and nonmetals. Silicon is a classic metalloid, along with boron, germanium, arsenic, antimony, and tellurium.

Question 31Structure of Matter

Which of the following molecules has a linear geometry?

  1. CO2 Correct answer.
  2. CH4
  3. H2O
  4. NH3

Explanation

Carbon dioxide has two electron domains around the central carbon (two double bonds) and no lone pairs, giving a linear shape with a 180° bond angle.

Question 32Structure of Matter

Which of the following bonds is the most polar?

  1. H–Br
  2. H–Cl
  3. H–F Correct answer.
  4. H–I

Explanation

Bond polarity increases with the electronegativity difference between the two atoms. Fluorine is the most electronegative element, so the H–F bond has the largest difference and is the most polar.

Question 33Structure of Matter

How many resonance structures can be drawn for the nitrate ion (NO3−)?

  1. 4
  2. 1
  3. 2
  4. 3 Correct answer.

Explanation

The nitrate ion has three equivalent oxygen atoms and one nitrogen. The double bond can be placed on any one of the three oxygens, giving three equivalent resonance structures.

Question 34Structure of Matter

Which type of crystal lattice is held together by a sea of delocalized electrons?

  1. Molecular
  2. Covalent network
  3. Ionic
  4. Metallic Correct answer.

Explanation

Metallic bonding involves positive metal ions surrounded by a delocalized sea of valence electrons. This model explains properties such as electrical conductivity and malleability.

Question 35Structure of Matter

Which of the following molecules is polar?

  1. BF3
  2. CCl4
  3. CH2Cl2 Correct answer.
  4. CO2

Explanation

Dichloromethane (CH2Cl2) has polar C–Cl and C–H bonds arranged tetrahedrally, and the bond dipoles do not cancel, so the molecule has a net dipole moment. The other molecules are highly symmetric and their bond dipoles cancel.

Question 36Structure of Matter

Which of the following pairs of elements would most likely form a covalent compound?

  1. C and O Correct answer.
  2. Mg and O
  3. K and F
  4. Na and Cl

Explanation

Covalent bonds typically form between nonmetals, which share electrons. Carbon and oxygen are both nonmetals, so they form covalent compounds such as CO2.

Question 37Structure of Matter

Which of the following statements correctly describes the trend in atomic radius as you move from left to right across a period?

  1. Atomic radius increases because more electron shells are added.
  2. Atomic radius remains constant because protons are added.
  3. Atomic radius decreases because electron shielding increases.
  4. Atomic radius decreases because effective nuclear charge increases. Correct answer.

Explanation

Across a period, protons are added to the nucleus while electrons enter the same principal energy level. The increasing effective nuclear charge pulls the electron cloud closer, so atomic radius decreases.

Question 38Structure of Matter

Which of the following ions has the largest ionic radius?

  1. Cl−
  2. K+
  3. P3− Correct answer.
  4. S2−

Explanation

These ions are isoelectronic, all having the electron configuration of argon. For isoelectronic species, the radius increases as the nuclear charge decreases because the electrons are held less tightly. Phosphorus has the smallest nuclear charge (Z=15), so P3− is the largest.

Question 39Structure of Matter

Which of the following molecules has a tetrahedral molecular geometry?

  1. NH3
  2. H2O
  3. SiH4 Correct answer.
  4. BF3

Explanation

Silane (SiH4) has four bonding pairs and no lone pairs around silicon, giving a tetrahedral electron-domain and molecular geometry.

Question 40Structure of Matter

Which of the following bonds is formed by the overlap of an s orbital and a p orbital?

  1. A pi bond in O2
  2. A sigma bond in Cl2
  3. A pi bond in N2
  4. A sigma bond in HF Correct answer.

Explanation

In hydrogen fluoride, the H–F sigma bond forms from the overlap of the hydrogen 1s orbital and a fluorine 2p orbital. Sigma bonds can involve s–s, s–p, or p–p overlap.

Question 41States of Matter

Which statement best describes the particles of a gas at a fixed temperature?

  1. They are widely separated and move randomly at high speed. Correct answer.
  2. They are closely packed but free to slide past one another.
  3. They are arranged in a repeating crystalline lattice.
  4. They are locked in fixed positions and vibrate in place.

Explanation

In a gas, intermolecular forces are overcome, so particles are far apart and travel in rapid random motion, colliding elastically. Fixed positions and lattices describe solids, while sliding close-packed particles describe liquids.

Question 42States of Matter

A sample of gas is compressed at constant temperature. According to Boyle's law, what happens to the pressure?

  1. It stays the same because temperature is constant.
  2. It increases in inverse proportion to the volume. Correct answer.
  3. It increases in direct proportion to the volume.
  4. It decreases in direct proportion to the volume.

Explanation

Boyle's law states that at constant temperature the product of pressure and volume is constant, so pressure and volume are inversely related. Halving the volume doubles the pressure.

Question 43States of Matter

A balloon filled with gas is warmed from 300 K to 600 K at constant pressure. What happens to its volume?

  1. It doubles. Correct answer.
  2. It halves.
  3. It quadruples.
  4. It stays the same.

Explanation

Charles's law states that volume is directly proportional to absolute temperature at constant pressure. Doubling the Kelvin temperature doubles the volume.

Question 44States of Matter

Which of the following is a unit of pressure equivalent to one atmosphere?

  1. 1 pascal
  2. 1 joule
  3. 1 mole
  4. 760 millimeters of mercury Correct answer.

Explanation

One atmosphere equals 760 mmHg (also 760 torr, or about 101.3 kPa). The pascal is the SI pressure unit but 1 Pa is far smaller than an atmosphere.

Question 45States of Matter

Which intermolecular force is primarily responsible for holding molecules of a liquid together so that it has a definite volume?

  1. Magnetic attraction between molecules
  2. Attraction between the molecules Correct answer.
  3. Gravitational attraction between molecules
  4. Repulsion between the nuclei

Explanation

In a liquid, intermolecular attractive forces are strong enough to keep molecules in contact but too weak to lock them into fixed positions. These attractions give the liquid a definite volume while allowing it to flow.

Question 46States of Matter

Which change of state involves a substance absorbing energy?

  1. Condensation
  2. Freezing
  3. Deposition
  4. Evaporation Correct answer.

Explanation

Evaporation (vaporization) converts a liquid to a gas and requires energy to overcome intermolecular attractions. Freezing, condensation, and deposition all release energy as particles come together.

Question 47States of Matter

What is the molar volume of an ideal gas at standard temperature and pressure (0 degrees Celsius, 1 atm)?

  1. 24.5 L
  2. 44.8 L
  3. 22.4 L Correct answer.
  4. 11.2 L

Explanation

At STP one mole of an ideal gas occupies 22.4 L, a value derived from the ideal gas law. This is the standard molar volume used in stoichiometric gas calculations.

Question 48States of Matter

Which property of a gas is defined as the force its particles exert per unit area on the walls of its container?

  1. Pressure Correct answer.
  2. Density
  3. Volume
  4. Temperature

Explanation

Pressure arises from gas particles colliding with container walls, and is defined as force per unit area. Temperature reflects average kinetic energy, volume is space occupied, and density is mass per volume.

Question 49States of Matter

A sample of gas has a pressure of 2.0 atm at 300 K. If the temperature is raised to 450 K at constant volume, what is the new pressure?

  1. 3.0 atm Correct answer.
  2. 1.3 atm
  3. 4.5 atm
  4. 2.0 atm

Explanation

Gay-Lussac's law states that pressure is directly proportional to absolute temperature at constant volume. The ratio 450/300 equals 1.5, so pressure becomes 2.0 atm times 1.5, or 3.0 atm.

Question 50States of Matter

A gas occupies 4.0 L at 1.5 atm. What volume will it occupy at 3.0 atm if the temperature is held constant?

  1. 8.0 L
  2. 6.0 L
  3. 2.0 L Correct answer.
  4. 4.0 L

Explanation

Boyle's law gives P1V1 = P2V2 at constant temperature. Here 1.5 atm times 4.0 L equals 6.0 atm-L, and dividing by 3.0 atm yields 2.0 L.