HESI A2 Chemistry Practice Test

25 practice questions

Choose an answer, then check it under Show answer.

1. Question 1 of 25Medium

   Which of the following represents the correct electron configuration for a neutral Sodium atom (atomic number 11)?

   1. AA. 1s2 2s2 2p6 3s2
   2. BB. 1s2 2s8
   3. CC. 1s2 2s2 2p6 3s1
   4. DD. 1s2 2s2 2p5
   Show answer to question 1

   Answer: C. 1s2 2s2 2p6 3s1

   Sodium has 11 electrons, which fill the 1s, 2s, and 2p subshells completely before the 3s orbital begins. The 1s2 2s2 2p6 configuration accounts for 10 electrons, leaving one electron for the 3s orbital.

2. Question 2 of 25Medium

   Paper chromatography separates the individual components of a liquid dye primarily based on differences in which characteristic?

   1. AA. Affinity for stationary and mobile phases
   2. BB. Rate of thermal expansion
   3. CC. Magnetic susceptibility
   4. DD. Melting point under atmospheric pressure
   Show answer to question 2

   Answer: A. Affinity for stationary and mobile phases

   Chromatography separates solutes according to their relative distribution between a stationary phase (such as paper) and a mobile phase (the solvent). Differences in melting point do not govern how solutes travel along chromatography paper.

3. Question 3 of 25Medium

   Potassium chlorate decomposes according to the balanced equation: 2 KClO3 -> 2 KCl + 3 O2. How many moles of potassium chlorate (KClO3) are required to produce 9.0 moles of oxygen gas (O2)?

   1. AA. 13.5 moles
   2. BB. 9.0 moles
   3. CC. 4.5 moles
   4. DD. 6.0 moles
   Show answer to question 3

   Answer: D. 6.0 moles

   The molar ratio between KClO3 and O2 is 2 to 3. Multiplying 9.0 moles of O2 by (2 moles KClO3 / 3 moles O2) yields 6.0 moles of KClO3. The option of 13.5 moles incorrectly inverts the ratio, multiplying by 3 and dividing by 2.

4. Question 4 of 25Easy

   An atom of an unknown element has 6 protons, 6 neutrons, and 6 electrons. What is the mass number of this atom?

   1. AA. 12
   2. BB. 6
   3. CC. 0
   4. DD. 18
   Show answer to question 4

   Answer: A. 12

   The mass number is the sum of the number of protons and neutrons in an atom's nucleus. Adding 6 protons and 6 neutrons results in a mass number of 12.

5. Question 5 of 25Easy

   According to Boyle's law, how does the volume of a gas change if the pressure applied to it is doubled at constant temperature?

   1. AA. It remains unchanged despite the increase in pressure.
   2. BB. It decreases to one-half of its original value.
   3. CC. It quadruples in size compared to its original value.
   4. DD. It doubles in size compared to its original value.
   Show answer to question 5

   Answer: B. It decreases to one-half of its original value.

   Boyle's law states that the pressure and volume of a gas are inversely proportional when temperature and mass are held constant; doubling the pressure reduces the volume to one-half. Doubling in size would represent a direct relationship, which contradicts Boyle's law.

6. Question 6 of 25Easy

   An atom of carbon has 6 protons, 6 neutrons, and 6 electrons. What is its atomic number?

   1. AA. 0
   2. BB. 12
   3. CC. 18
   4. DD. 6
   Show answer to question 6

   Answer: D. 6

   The atomic number is defined as the number of protons in an atom's nucleus. For carbon, this is 6.

   Why the other options are wrong

   • A0 is incorrect; an atom must have at least one proton to be an element.
   • B12 is the mass number (protons + neutrons), not the atomic number.
   • C18 is the sum of all subatomic particles (protons + neutrons + electrons), which is not the atomic number.

7. Question 7 of 25Easy

   Which combination of elements is most likely to form an ionic compound?

   1. AA. Carbon and oxygen
   2. BB. Potassium and chlorine
   3. CC. Sulfur and fluorine
   4. DD. Nitrogen and hydrogen
   Show answer to question 7

   Answer: B. Potassium and chlorine

   Ionic bonds typically form between a metal with low electronegativity and a nonmetal with high electronegativity, such as potassium and chlorine forming potassium chloride. Pairs consisting entirely of nonmetals, such as carbon and oxygen, share electrons to form covalent bonds.

8. Question 8 of 25Easy

   Which of the following is an example of a physical change?

   1. AA. The souring of milk due to bacteria
   2. BB. The melting of an ice cube
   3. CC. The rusting of an iron nail
   4. DD. A piece of wood burning in a fireplace
   Show answer to question 8

   Answer: B. The melting of an ice cube

   Melting is a change of state, which is a physical process that does not alter the chemical identity of the substance. Burning, rusting, and souring all involve chemical reactions that produce new substances.

9. Question 9 of 25Easy

   Which observation provides the strongest evidence that a chemical change has occurred when mixing two clear liquids at room temperature?

   1. AA. The rapid formation of an insoluble solid precipitate
   2. BB. The uninterrupted transmission of light through the mixture
   3. CC. A slight decrease in the total volume of the mixture
   4. DD. The uniform mechanical dispersal of the liquid molecules
   Show answer to question 9

   Answer: A. The rapid formation of an insoluble solid precipitate

   The spontaneous formation of an insoluble solid precipitate indicates that a reaction took place to generate a new substance with different solubility characteristics. Slight volume shifts or mechanical mixing can happen during purely physical mixing without the breaking or forming of chemical bonds.

10. Question 10 of 25Medium

    When a neutral calcium atom loses two valence electrons to form a calcium ion (Ca2+), what effect does this change have on its atomic number and mass number?

    1. AA. Both the atomic number and mass number decrease by two.
    2. BB. The atomic number remains unchanged, but the mass number decreases by two.
    3. CC. The atomic number decreases by two, but the mass number remains unchanged.
    4. DD. Both the atomic number and mass number remain unchanged.
    Show answer to question 10

    Answer: D. Both the atomic number and mass number remain unchanged.

    The atomic number depends solely on the number of nuclear protons, and the mass number reflects the combined total of protons and neutrons. Because ionization involves only the loss or gain of extranuclear electrons, the nuclear composition is unaltered and both numbers stay the same.

11. Question 11 of 25Easy

    Which of the following household substances typically has a pH greater than 7.0 at 25 °C?

    1. AA. Household bleach
    2. BB. Pure distilled water
    3. CC. Black coffee
    4. DD. Lemon juice
    Show answer to question 11

    Answer: A. Household bleach

    Household bleach is an alkaline solution containing sodium hypochlorite, giving it a pH well above 7.0 (typically around 11 to 13). Black coffee and lemon juice are acidic with pH values below 7.0, while pure distilled water is neutral with a pH of exactly 7.0 at 25 °C.

12. Question 12 of 25Medium

    A bond between oxygen (electronegativity 3.44) and hydrogen (2.20) is classified as which type?

    1. AA. Ionic
    2. BB. Metallic
    3. CC. Polar covalent
    4. DD. Nonpolar covalent
    Show answer to question 12

    Answer: C. Polar covalent

    The electronegativity difference is 3.44 - 2.20 = 1.24. A difference between about 0.5 and 1.7 indicates a polar covalent bond, in which electrons are shared unequally and oxygen carries a partial negative charge.

    Why the other options are wrong

    • AIonic bonds involve electron transfer and usually require a difference greater than about 1.7; 1.24 falls short.
    • BMetallic bonds form among metal atoms sharing a sea of electrons; oxygen and hydrogen are both nonmetals.
    • DNonpolar covalent bonds have a difference below about 0.5, meaning nearly equal sharing; 1.24 is too large.

13. Question 13 of 25Medium

    A metal cylinder has a mass of 45.2 g and a volume of 5.8 mL. What is its density reported to the correct number of significant figures?

    1. AA. 7.8 g/mL
    2. BB. 7.79 g/mL
    3. CC. 7.793 g/mL
    4. DD. 8 g/mL
    Show answer to question 13

    Answer: A. 7.8 g/mL

    In multiplication and division, the final answer must have the same number of significant figures as the measurement with the fewest significant figures. The mass has three significant figures and the volume has two, so the calculated density of 7.7931 g/mL must be rounded to two significant figures, giving 7.8 g/mL. Keeping three figures as 7.79 g/mL ignores the two significant figure limit of the volume.

14. Question 14 of 25Medium

    Which statement best describes the relationship between a solute and a solvent in a dilute solution?

    1. AA. The solvent is the substance being dissolved
    2. BB. The solute and solvent must be in the same physical state
    3. CC. The solute is present in a much larger quantity than the solvent
    4. DD. The solute is present in a much smaller quantity than the solvent
    Show answer to question 14

    Answer: D. The solute is present in a much smaller quantity than the solvent

    In a dilute solution, the solute is present in a low concentration relative to the solvent. The solvent is the component present in the largest amount.

15. Question 15 of 25Hard

    An unknown solid sample cannot be separated into simpler components by any physical technique, but it decomposes into two distinct gases when exposed to an electric current. Which term correctly classifies the original solid?

    1. AA. Homogeneous mixture
    2. BB. Compound
    3. CC. Heterogeneous mixture
    4. DD. Element
    Show answer to question 15

    Answer: B. Compound

    A compound is a pure substance that cannot be resolved into simpler parts by physical methods, but can be broken down into constituent elements by chemical reactions such as electrolysis. An element cannot be broken down by chemical processes, while mixtures are physically separable without breaking chemical bonds.

16. Question 16 of 25Medium

    How many milliliters of a 4.0 M HCl solution are needed to prepare 200 mL of 0.5 M HCl?

    1. AA. 10 mL
    2. BB. 100 mL
    3. CC. 25 mL
    4. DD. 50 mL
    Show answer to question 16

    Answer: C. 25 mL

    Use M₁V₁ = M₂V₂: V₁ = (0.5 M × 200 mL) ÷ 4.0 M = 100 ÷ 4.0 = 25 mL of the 4.0 M stock.

    Why the other options are wrong

    • A10 mL of 4.0 M stock diluted to 200 mL gives only 0.2 M.
    • B100 mL of stock diluted to 200 mL gives 2.0 M, four times the target.
    • D50 mL of stock diluted to 200 mL gives 1.0 M, twice the target.

17. Question 17 of 25Medium

    Which element in Period 3 has the highest first ionization energy?

    1. AA. P (phosphorus)
    2. BB. Na (sodium)
    3. CC. Al (aluminum)
    4. DD. Ar (argon)
    Show answer to question 17

    Answer: D. Ar (argon)

    First ionization energy generally rises from left to right across a period because nuclear charge increases while atomic radius shrinks. Argon, the noble gas at the end of Period 3, has a full valence shell and the highest first ionization energy in the period.

    Why the other options are wrong

    • APhosphorus (group 15) sits left of argon, so its outer electrons are held less tightly and are easier to remove.
    • BSodium has a single 3s valence electron that is easily removed, giving it the lowest first ionization energy in Period 3.
    • CAluminum loses its lone 3p electron easily, so its first ionization energy is among the lowest in Period 3.

18. Question 18 of 25Medium

    Which of the following particles is considered electrically neutral?

    1. AA. Electron
    2. BB. Proton
    3. CC. Positron
    4. DD. Neutron
    Show answer to question 18

    Answer: D. Neutron

    Neutrons have no electrical charge, making them neutral. Protons carry a positive charge, and electrons carry a negative charge.

19. Question 19 of 25Hard

    What is the correct chemical formula for Aluminum Oxide?

    1. AA. AlO
    2. BB. Al2O
    3. CC. Al2O3
    4. DD. AlO2
    Show answer to question 19

    Answer: C. Al2O3

    Aluminum has a charge of +3 and oxygen has a charge of -2; therefore, the smallest ratio to balance the charges is two aluminum ions for every three oxygen ions, forming Al2O3. AlO2 and other ratios do not result in a neutral compound.

20. Question 20 of 25Medium

    How many moles are contained in 44 grams of carbon dioxide (CO2)? (Atomic weights: C = 12, O = 16)

    1. AA. 0.5 moles
    2. BB. 2.0 moles
    3. CC. 1.0 moles
    4. DD. 4.0 moles
    Show answer to question 20

    Answer: C. 1.0 moles

    The molar mass of CO2 is 44 g/mol (12 + 16 + 16). Dividing the given mass of 44 grams by the molar mass of 44 g/mol yields exactly 1.0 mole.

21. Question 21 of 25Hard

    An element has two naturally occurring isotopes: X‑20 with a mass of 20.000 amu (90% abundance) and X‑22 with a mass of 22.000 amu (10% abundance). What is the average atomic mass of element X?

    1. AA. 20.2 amu
    2. BB. 21.0 amu
    3. CC. 20.0 amu
    4. DD. 22.0 amu
    Show answer to question 21

    Answer: A. 20.2 amu

    Average atomic mass = (fraction₁ × mass₁) + (fraction₂ × mass₂) = (0.90 × 20.000) + (0.10 × 22.000) = 18.0 + 2.2 = 20.2 amu.

    Why the other options are wrong

    • B21.0 amu is the simple arithmetic mean of 20 and 22, not weighted by abundance.
    • C20.0 amu is the mass of the more abundant isotope alone, ignoring the heavier isotope.
    • D22.0 amu is the mass of the less abundant isotope alone.

22. Question 22 of 25Hard

    Which property of water is primarily due to hydrogen bonding between water molecules?

    1. AA. Low specific heat capacity
    2. BB. High surface tension
    3. CC. Ability to dissolve nonpolar substances
    4. DD. Inability to form ions
    Show answer to question 22

    Answer: B. High surface tension

    Hydrogen bonds create a strong cohesive force at the water surface, resulting in high surface tension. Water does not dissolve nonpolar substances (it is a polar solvent), has a high specific heat (not low), and can autoionize into H⁺ and OH⁻ ions.

    Why the other options are wrong

    • AWater has a high specific heat capacity, not low, due to hydrogen bonds, but the option states 'low' which is incorrect.
    • CWater is a polar solvent and does not effectively dissolve nonpolar substances; that property is not due to hydrogen bonding.
    • DWater can form ions through autoionization (H₂O ⇌ H⁺ + OH⁻), so it does have the ability to form ions.

23. Question 23 of 25Hard

    According to the balanced reaction 2H2 + O2 -> 2H2O, how many grams of water are produced when 0.50 moles of oxygen gas (O2) react completely with excess hydrogen? (Molar mass of H2O = 18.02 g/mol)

    1. AA. 9.01 g
    2. BB. 27.03 g
    3. CC. 18.02 g
    4. DD. 36.04 g
    Show answer to question 23

    Answer: C. 18.02 g

    The mole ratio of O2 to H2O is 1:2, so 0.50 moles of O2 will produce 1.00 mole of H2O. Multiplying 1.00 mole by the molar mass of water (18.02 g/mol) results in 18.02 g. The value of 9.01 g results from failing to apply the 1:2 stoichiometric mole ratio and multiplying 0.50 moles directly by the molar mass.

24. Question 24 of 25Hard

    What is the molar mass of magnesium sulfate heptahydrate, MgSO4 · 7H2O? (Atomic masses: Mg = 24.31 g/mol, S = 32.06 g/mol, O = 16.00 g/mol, H = 1.01 g/mol)

    1. AA. 262.51 g/mol
    2. BB. 246.51 g/mol
    3. CC. 968.73 g/mol
    4. DD. 138.39 g/mol
    Show answer to question 24

    Answer: B. 246.51 g/mol

    Anhydrous MgSO4 has a mass of 24.31 + 32.06 + (4 x 16.00) = 120.37 g/mol. Seven water molecules add 7 x [(2 x 1.01) + 16.00] = 7 x 18.02 = 126.14 g/mol, giving a total molar mass of 120.37 + 126.14 = 246.51 g/mol. The value 138.39 g/mol is incorrect because it accounts for only one water molecule rather than seven.

25. Question 25 of 25Hard

    If 25 mL of a 0.80 M potassium chloride solution is diluted by adding 75 mL of distilled water, what is the final concentration of the solution?

    1. AA. 0.40 M
    2. BB. 0.25 M
    3. CC. 0.60 M
    4. DD. 0.20 M
    Show answer to question 25

    Answer: D. 0.20 M

    The total final volume is the sum of the initial volume and the volume added (25 mL + 75 mL = 100 mL). Applying M1V1 = M2V2, (0.80 M)(25 mL) = M2(100 mL), which results in 0.20 M. A common mistake is using the added volume (75 mL) as the final volume.

Email me my score report

The full HESI A2 bank has 415 Chemistry questions. See plans