HESI A2 Chemistry Practice Questions

Correct Answer: B

Rationale: The correct answer is B (H₂SO₄) because it is a strong acid that completely dissociates into ions in water, leading to a high concentration of H⁺ ions. This is indicated by its ability to donate multiple protons. Choice A (CH₃COOH) is a weak acid that only partially dissociates. Choice C (NH₃) is a weak base that accepts protons rather than donating them. Choice D (KOH) is a strong base that dissociates completely in water but is not an acid.

Correct Answer: A

Rationale: The correct answer is A: 2. Helium-4 has an atomic number of 2, which represents the number of protons in its nucleus. Since the atomic number is the sum of protons and neutrons, and helium-4 has 2 protons, it must also have 2 neutrons to make the total atomic mass 4. Therefore, there are 2 neutrons in an atom of helium-4. Choice B: 3 is incorrect because it does not match the atomic number of helium-4. Choice C: 4 is incorrect as it is the total atomic mass of helium-4, not the number of neutrons. Choice D: 6 is incorrect as it does not correspond to the atomic structure of helium-4.

Correct Answer: A

Rationale: Answer A (NH₃) does not name a polar molecule because it has a trigonal pyramidal shape with a lone pair on nitrogen, causing an uneven distribution of electron density. This results in a net dipole moment, making NH₃ a polar molecule. In contrast, choices B (H₂S), C (SO₂), and D (CO₂) all have symmetrical shapes or polar bonds that cancel out the dipole moments, making them nonpolar molecules.

Correct Answer: C

Rationale: Failed to generate a rationale after 5 retries.

Correct Answer: B

Rationale: In a single displacement reaction, an active element displaces a less active element in a compound, resulting in the formation of a new compound. This is because more reactive elements have a higher tendency to replace less reactive elements in a compound. The other choices are incorrect because: A: A compound decomposing into two substances describes a decomposition reaction, not a single displacement reaction. C: Formation of a precipitate from the reaction of two solutions describes a double displacement reaction, not a single displacement reaction. D: The change in oxidation states of atoms in the reactants is not a defining characteristic of a single displacement reaction.