Free Praxis® Chemistry Practice Questions

Sample Practice Questions

Try these 10 sample questions from the Praxis® Chemistry practice bank. Each includes the answer and explanation. Continue with the interactive practice above to review the full question set.

1. 1Students compare reaction times at four temperatures, but use a different reactant concentration at each temperature. Why does this design fail to isolate the effect of temperature?

   1. A.Temperature is measured rather than manipulated.
   2. B.Concentration provides a confounding variable.Correct answer: B
   3. C.Reaction time must remain constant in every trial.
   4. D.Four temperatures prevent a controlled comparison.

   Explanation: ETS advises, "Consider possible confounding variables or effects and evaluate the investigation’s design to ensure variables are controlled." Changing concentration together with temperature makes their effects inseparable.

   Reference: ETS Chemistry Study Companion — p. 16, SEP 3

2. 2Which particle count determines the chemical identity of an element?

   1. A.Number of neutrons
   2. B.Number of nucleons
   3. C.Number of protonsCorrect answer: C
   4. D.Number of occupied orbitals

   Explanation: DOE states, "An atom is classified chemically by the number of protons in its nucleus." Neutrons distinguish isotopes; nucleon and orbital counts do not uniquely identify an element.

   Reference: DOE Fundamentals Handbook: Chemistry, Volume 1 — CH-01, p. 3, Chemical Elements

3. 3Approximately how many molecules are in 0.50 mol of a molecular substance? Use NA = 6.022 × 10²³ mol⁻¹.

   1. A.1.20 × 10²⁴
   2. B.3.01 × 10²³Correct answer: B
   3. C.6.02 × 10²³
   4. D.8.30 × 10⁻²⁵

   Explanation: NIST states, "One mole (mol) contains exactly 6.02214076 × 1023 elementary entities." Using the approximate NA given in the stem, N = nNA = 0.50 × 6.022 × 10²³ = 3.01 × 10²³ molecules.

   Reference: SI Units – Amount of Substance — Amount of substance definition

4. 4What quantity determines the order of elements in the modern periodic table?

   1. A.Atomic radius
   2. B.Neutron count
   3. C.Atomic numberCorrect answer: C
   4. D.Most abundant isotope mass

   Explanation: The handbook says elements are "arranged in order of increasing atomic number". Radius, isotope mass, and neutron count do not define the ordering.

   Reference: DOE Fundamentals Handbook: Chemistry, Volume 1 — CH-01, p. 14

5. 5Which description defines a solution and distinguishes it from a compound or a heterogeneous mixture?

   1. A.A substance with a fixed ratio of chemically bonded elements
   2. B.A mixture containing particles that settle on standing
   3. C.A homogeneous mixture of two or more substancesCorrect answer: C
   4. D.A pure substance containing only one kind of atom

   Explanation: The handbook states, "A homogeneous mixture of two or more substances is called a solution." Solutions have uniform composition but need not have the fixed composition of a compound.

   Reference: DOE Fundamentals Handbook: Chemistry, Volume 1 — CH-01, p. 33, Combining Elements

6. 6Which hypothesis states a directional relationship that students could test by changing the temperature of water and measuring sugar dissolution time?

   1. A.Dissolution time may change when water temperature changes.
   2. B.Faster stirring will decrease the time needed to dissolve sugar.
   3. C.Warmer water will decrease the time needed to dissolve a fixed sugar mass.Correct answer: C
   4. D.Warmer water will increase the maximum mass of sugar that dissolves.

   Explanation: ETS specifies, "Make directional hypotheses that specify what happens to a dependent variable when an independent variable is manipulated." The prediction identifies a change in measured time as temperature changes.

   Reference: ETS Chemistry Study Companion — p. 16, SEP 3

7. 7An atom has atomic number 19 and mass number 41. How many neutrons are in its nucleus?

   1. A.19
   2. B.22Correct answer: B
   3. C.41
   4. D.60

   Explanation: DOE defines mass number as "The sum of the total number of protons, Z, and the total number of neutrons, N". Therefore N = 41 − 19 = 22.

   Reference: DOE Fundamentals Handbook: Chemistry, Volume 1 — CH-01, p. 4, Atomic Mass Number

8. 8Using Ca = 40, C = 12, and O = 16 g/mol, what is the molar mass of CaCO3?

   1. A.68 g/mol
   2. B.84 g/mol
   3. C.100 g/molCorrect answer: C
   4. D.204 g/mol

   Explanation: DOE explains that "the atomic masses of all atoms that appear in the formula are added." Thus 40 + 12 + 3(16) = 100 g/mol.

   Reference: DOE Fundamentals Handbook: Chemistry, Volume 1 — CH-01, p. 10, Mole of Molecules

9. 9Two main-group elements have closely similar chemical properties and occupy the same vertical column. What is this column called?

   1. A.A period
   2. B.A groupCorrect answer: B
   3. C.An isotope series
   4. D.A mass-number sequence

   Explanation: The source states, "Each set of elements appearing in the vertical column of a periodic table is called a Group". A period is a horizontal row; isotope series concern one element.

   Reference: DOE Fundamentals Handbook: Chemistry, Volume 1 — CH-01, p. 18, Group Characteristics

10. 10Which assignment correctly identifies the solvent and solute in a dilute aqueous glucose solution?

    1. A.Water is the solute; glucose is the solvent.
    2. B.Water is the solvent; glucose is the solute.Correct answer: B
    3. C.Both are solutes; the solution has no solvent.
    4. D.Both are solvents; the solution has no solute.

    Explanation: The handbook states: "The solvent is the material that dissolves the other substance(s)." Water provides the dissolving medium, and glucose is dissolved in it. Thus water is the solvent and glucose is the solute.

    Reference: DOE Fundamentals Handbook: Chemistry, Volume 1 — CH-01, p. 33, Combining Elements