22 Chemistry Practice Questions & Answers

Every Chemistry practice question from the HESI A2 Practice Test, with the correct answer and a short explanation.

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1. Normal body temperature is about 37 °C. Using K = °C + 273, what is this temperature in kelvins?
- A.373 K
- B.310 K✓ Answer
- C.98.6 K
- D.236 K
To convert Celsius to Kelvin you add 273, so 37 + 273 = 310 K. Subtracting 273 gives the impossible 236 K, 98.6 is the Fahrenheit value, and 373 K is the boiling point of water.
Ref QB-VAU6VDSource: Elsevier HESI A2 — Chemistry: temperature scales (K = °C + 273)Report a problem with this question
2. Solid carbon dioxide (dry ice) changes directly into a gas without first melting into a liquid. What is this phase change called?
- A.Evaporation
- B.Deposition
- C.Condensation
- D.Sublimation✓ Answer
Sublimation is the direct change from solid to gas, skipping the liquid state. Deposition is the reverse (gas to solid), evaporation is liquid to gas, and condensation is gas to liquid.
Ref QB-CTNZ9DSource: Elsevier HESI A2 — Chemistry: states of matter and phase changesReport a problem with this question
3. A calcium ion, Ca²⁺, has atomic number 20 and mass number 40. How many protons, neutrons, and electrons does this ion have?
- A.18 protons, 22 neutrons, 18 electrons
- B.20 protons, 20 neutrons, 18 electrons✓ Answer
- C.20 protons, 20 neutrons, 22 electrons
- D.20 protons, 40 neutrons, 18 electrons
The atomic number fixes the protons at 20, and neutrons = mass number − atomic number = 40 − 20 = 20. A 2+ charge means the atom lost two electrons, leaving 20 − 2 = 18. Forming an ion changes only the electron count, never the protons.
Ref QB-UAGUPCSource: Elsevier HESI A2 — Chemistry: atomic structure (ions and neutron count)Report a problem with this question
4. Chlorine-37 has an atomic number of 17. How many neutrons are in one atom of chlorine-37?
- A.20✓ Answer
- B.17
- C.54
- D.37
The number in an isotope's name is its mass number (protons + neutrons). Neutrons = 37 − 17 = 20. Seventeen is the proton count, 37 is the mass number itself, and 54 comes from wrongly adding the two numbers.
Ref QB-FTQZSGSource: Elsevier HESI A2 — Chemistry: atomic structure (neutrons = mass number − atomic number)Report a problem with this question
5. Which of these elements has the highest electronegativity?
- A.Chlorine
- B.Oxygen
- C.Fluorine✓ Answer
- D.Nitrogen
Electronegativity increases across a period from left to right and decreases down a group. Fluorine sits at the top right of the table (noble gases excluded), so it is the most electronegative element. Chlorine is below it, and oxygen and nitrogen are to its left.
Ref QB-XAJU7GSource: Elsevier HESI A2 — Chemistry: periodic trends (electronegativity)Report a problem with this question
6. Sodium (Na), magnesium (Mg), and chlorine (Cl) are in period 3, and potassium (K) lies directly below sodium in period 4. Which atom has the largest atomic radius?
- A.Potassium✓ Answer
- B.Chlorine
- C.Sodium
- D.Magnesium
Atomic radius increases down a group because each period adds an energy level, and it decreases from left to right because more protons pull the electrons closer. Potassium is both farther down and at the far left, so it is the largest; chlorine is the smallest of the four.
Ref QB-ETCPMFSource: Elsevier HESI A2 — Chemistry: periodic trends (atomic radius)Report a problem with this question
7. Which family of elements consists of soft, highly reactive metals that typically form ions with a +1 charge?
- A.Transition metals
- B.Alkaline earth metals
- C.Halogens
- D.Alkali metals✓ Answer
Alkali metals (Group 1) have one valence electron, which they lose easily to form +1 ions; this makes them extremely reactive. Alkaline earth metals (Group 2) form +2 ions, halogens (Group 17) are nonmetals that gain one electron to form −1 ions, and transition metals are less reactive and often have several possible charges.
Ref QB-WA7H2FSource: Elsevier HESI A2 — Chemistry: periodic table families (Group 1 alkali metals)Report a problem with this question
8. Which substance contains a nonpolar covalent bond?
- A.Cl₂✓ Answer
- B.H₂O
- C.HCl
- D.NaCl
In Cl₂ the two bonded atoms are identical, so their electronegativity difference is zero and the electrons are shared equally, making the bond nonpolar covalent. HCl and H₂O have polar covalent bonds because the atoms differ in electronegativity, and NaCl is ionic because sodium transfers an electron to chlorine.
Ref QB-HTEP5KSource: Elsevier HESI A2 — Chemistry: bonding (polar vs nonpolar covalent)Report a problem with this question
9. Which substance forms hydrogen bonds between its molecules?
- A.NH₃✓ Answer
- B.H₂S
- C.HCl
- D.CH₄
Hydrogen bonding occurs only when hydrogen is covalently bonded to nitrogen, oxygen, or fluorine, the small, highly electronegative atoms. NH₃ has H bonded to N. CH₄, H₂S, and HCl contain hydrogen, but it is bonded to carbon, sulfur, or chlorine, which do not produce hydrogen bonds.
Ref QB-ZA9HJJSource: Elsevier HESI A2 — Chemistry: intermolecular forces (hydrogen bonding with N, O, F)Report a problem with this question
10. What coefficient belongs in the blank to balance this equation? ___ Al + 3 O₂ → 2 Al₂O₃
- A.3
- B.4✓ Answer
- C.6
- D.2
The products contain 2 × 2 = 4 aluminum atoms and 2 × 3 = 6 oxygen atoms. The 3 O₂ already supplies 6 oxygen atoms, so the left side needs 4 Al to match the 4 Al on the right. Atoms are conserved, so every element must be equal on both sides.
Ref QB-JZWZ53Source: Elsevier HESI A2 — Chemistry: balancing chemical equations (conservation of mass)Report a problem with this question
11. Zn + CuSO₄ → ZnSO₄ + Cu. Which type of reaction is this?
- A.Decomposition
- B.Single replacement✓ Answer
- C.Synthesis
- D.Double replacement
The pattern A + BC → AC + B is single replacement: one free element (zinc) takes the place of another (copper) in a compound, which happens because zinc is more active than copper. Double replacement swaps partners between two compounds, decomposition breaks one compound apart, and synthesis combines substances into one product.
Ref QB-3H47R3Source: Elsevier HESI A2 — Chemistry: reaction types (single replacement, A + BC → AC + B)Report a problem with this question
12. How many moles are in 36 g of water (H₂O)? Use H = 1 g/mol and O = 16 g/mol.
- A.648 mol
- B.2.0 mol✓ Answer
- C.0.5 mol
- D.18 mol
The molar mass of H₂O is 2(1) + 16 = 18 g/mol. Moles = mass ÷ molar mass = 36 g ÷ 18 g/mol = 2.0 mol. Dividing the wrong way gives 0.5, 18 is only the molar mass, and 648 comes from multiplying instead of dividing.
Ref QB-HZJPYZSource: Elsevier HESI A2 — Chemistry: stoichiometry (moles = mass ÷ molar mass)Report a problem with this question
13. N₂ + 3 H₂ → 2 NH₃. How many moles of NH₃ form when 6 moles of H₂ react completely with excess N₂?
- A.2 mol
- B.12 mol
- C.9 mol
- D.4 mol✓ Answer
The balanced coefficients give a mole ratio of 3 mol H₂ : 2 mol NH₃. So 6 mol H₂ × (2 mol NH₃ / 3 mol H₂) = 4 mol NH₃. Inverting the ratio gives 9, multiplying 6 by the coefficient 2 alone gives 12, and 2 mol simply copies the NH₃ coefficient.
Ref QB-2HRUJ2Source: Elsevier HESI A2 — Chemistry: stoichiometry (mole ratios from balanced equations)Report a problem with this question
14. What is the oxidation number of sulfur in sulfuric acid, H₂SO₄?
- A.+2
- B.−2
- C.+6✓ Answer
- D.+4
In a neutral compound the oxidation numbers add to zero. Hydrogen is +1 and oxygen is −2, so 2(+1) + S + 4(−2) = 0, which gives S = +6. The value −2 is sulfur's charge as a simple sulfide ion, not its state in this compound.
Ref QB-GZ7CTYSource: Elsevier HESI A2 — Chemistry: oxidation-number rules (sum = 0 in a neutral compound)Report a problem with this question
15. In the reaction 2 Mg + O₂ → 2 MgO, which statement is correct?
- A.Mg is reduced, and O₂ is the reducing agent.
- B.Mg is oxidized, and O₂ is the oxidizing agent.✓ Answer
- C.Mg is oxidized, and Mg is the oxidizing agent.
- D.O₂ is oxidized, and Mg is the oxidizing agent.
Magnesium goes from 0 to +2, so it loses electrons and is oxidized (OIL RIG: oxidation is loss). Oxygen goes from 0 to −2, so it gains electrons and is reduced. The species that is reduced is the oxidizing agent, so O₂ is the oxidizing agent and Mg is the reducing agent.
Ref QB-ZGDHDZSource: Elsevier HESI A2 — Chemistry: oxidation-reduction (OIL RIG; oxidizing agent is reduced)Report a problem with this question
16. A solution has a hydrogen ion concentration of 1 × 10⁻⁴ M. What is its pH, and how is it classified?
- A.pH 4, acidic✓ Answer
- B.pH 4, basic
- C.pH 10, acidic
- D.pH 10, basic
pH = −log[H⁺], so [H⁺] = 1 × 10⁻⁴ M gives pH = 4. Any pH below 7 is acidic. A pH of 10 would be the pOH here (14 − 4), not the pH, and would describe a basic solution.
Ref QB-FZUZMXSource: Elsevier HESI A2 — Chemistry: acids and bases (pH = −log[H⁺])Report a problem with this question
17. In the reaction NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, how does water behave according to the Brønsted-Lowry definition?
- A.As an acid, because it accepts a proton
- B.As a base, because it donates a proton
- C.As a base, because it accepts a proton
- D.As an acid, because it donates a proton✓ Answer
A Brønsted-Lowry acid donates a proton (H⁺) and a base accepts one. Water gives up an H⁺ and becomes OH⁻, so it acts as the acid, while NH₃ accepts that proton to become NH₄⁺ and acts as the base.
Ref QB-YG278YSource: Elsevier HESI A2 — Chemistry: acids and bases (Brønsted-Lowry proton donor/acceptor)Report a problem with this question
18. Uranium-238 (atomic number 92) undergoes alpha decay. What are the mass number and atomic number of the product nucleus?
- A.Mass number 234, atomic number 90✓ Answer
- B.Mass number 234, atomic number 94
- C.Mass number 238, atomic number 93
- D.Mass number 236, atomic number 91
An alpha particle is a helium nucleus with 2 protons and 2 neutrons (mass 4, atomic number 2). Emitting one lowers the mass number by 4 (238 → 234) and the atomic number by 2 (92 → 90), since both totals are conserved in a nuclear equation. Mass 238 with atomic number 93 describes beta decay instead.
Ref QB-PZBCU7Source: Elsevier HESI A2 — Chemistry: nuclear chemistry (alpha decay, ⁴₂He)Report a problem with this question
19. Carbon-14 (atomic number 6) undergoes beta (β⁻) decay. What are the mass number and atomic number of the product nucleus?
- A.Mass number 10, atomic number 4
- B.Mass number 14, atomic number 5
- C.Mass number 14, atomic number 7✓ Answer
- D.Mass number 13, atomic number 6
In beta decay a neutron changes into a proton and an electron is emitted from the nucleus. The atomic number rises by 1 (6 → 7, nitrogen) while the mass number stays at 14, because the total of protons plus neutrons is unchanged. A decrease to atomic number 5 is the direction of positron emission, and losing 4 mass units is alpha decay.
Ref QB-7HHHE8Source: Elsevier HESI A2 — Chemistry: nuclear chemistry (beta decay, atomic number +1)Report a problem with this question
20. A radioactive isotope has a half-life of 8 days. What fraction of the original sample remains after 24 days?
- A.1/4
- B.1/6
- C.1/8✓ Answer
- D.1/3
24 days ÷ 8 days = 3 half-lives, and each half-life halves the amount, so the fraction left is (1/2)³ = 1/8. Using 1/3 or 1/6 treats decay as linear instead of repeated halving, and 1/4 stops after only two half-lives.
Ref QB-FXWUMXSource: Elsevier HESI A2 — Chemistry: nuclear chemistry (half-life, fraction remaining = (1/2)^n)Report a problem with this question
21. What is the molarity of a solution made by dissolving 0.50 mol of NaCl in enough water to make 250 mL of solution?
- A.125 M
- B.0.125 M
- C.0.50 M
- D.2.0 M✓ Answer
Molarity is moles of solute per liter of solution, so the volume must first be converted: 250 mL = 0.250 L. Then M = 0.50 mol ÷ 0.250 L = 2.0 M. Using 0.50 M ignores the volume, 0.125 M multiplies instead of divides, and 125 M multiplies by milliliters.
Ref QB-XERP2XSource: Elsevier HESI A2 — Chemistry: solutions (molarity = moles solute / liters solution)Report a problem with this question
22. Which statement is true of a reversible reaction that has reached chemical equilibrium?
- A.Reactant and product concentrations are equal.
- B.All reactants have been converted to products.
- C.The forward and reverse reaction rates are equal.✓ Answer
- D.The forward and reverse reactions have stopped.
Equilibrium is dynamic: both reactions keep running, but at the same rate, so the concentrations stay constant. Constant does not mean equal, because reactant and product concentrations can differ greatly at equilibrium, and the reaction never stops or runs to completion.
Ref QB-EXJHFWSource: Elsevier HESI A2 — Chemistry: reaction rates and equilibrium (dynamic equilibrium, equal forward/reverse rates)Report a problem with this question

1. Normal body temperature is about 37 °C. Using K = °C + 273, what is this temperature in kelvins?

2. Solid carbon dioxide (dry ice) changes directly into a gas without first melting into a liquid. What is this phase change called?

3. A calcium ion, Ca²⁺, has atomic number 20 and mass number 40. How many protons, neutrons, and electrons does this ion have?

4. Chlorine-37 has an atomic number of 17. How many neutrons are in one atom of chlorine-37?

5. Which of these elements has the highest electronegativity?

6. Sodium (Na), magnesium (Mg), and chlorine (Cl) are in period 3, and potassium (K) lies directly below sodium in period 4. Which atom has the largest atomic radius?

7. Which family of elements consists of soft, highly reactive metals that typically form ions with a +1 charge?

8. Which substance contains a nonpolar covalent bond?

9. Which substance forms hydrogen bonds between its molecules?

10. What coefficient belongs in the blank to balance this equation? ___ Al + 3 O₂ → 2 Al₂O₃

11. Zn + CuSO₄ → ZnSO₄ + Cu. Which type of reaction is this?

12. How many moles are in 36 g of water (H₂O)? Use H = 1 g/mol and O = 16 g/mol.

13. N₂ + 3 H₂ → 2 NH₃. How many moles of NH₃ form when 6 moles of H₂ react completely with excess N₂?

14. What is the oxidation number of sulfur in sulfuric acid, H₂SO₄?

15. In the reaction 2 Mg + O₂ → 2 MgO, which statement is correct?

16. A solution has a hydrogen ion concentration of 1 × 10⁻⁴ M. What is its pH, and how is it classified?

17. In the reaction NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, how does water behave according to the Brønsted-Lowry definition?

18. Uranium-238 (atomic number 92) undergoes alpha decay. What are the mass number and atomic number of the product nucleus?

19. Carbon-14 (atomic number 6) undergoes beta (β⁻) decay. What are the mass number and atomic number of the product nucleus?

20. A radioactive isotope has a half-life of 8 days. What fraction of the original sample remains after 24 days?

21. What is the molarity of a solution made by dissolving 0.50 mol of NaCl in enough water to make 250 mL of solution?

22. Which statement is true of a reversible reaction that has reached chemical equilibrium?

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