HESI A2 Chemistry Practice Questions

Correct Answer: A

Rationale: Step-by-step rationale: 1. Heating increases kinetic energy of particles. 2. Higher kinetic energy results in faster movement. 3. Faster movement leads to increased collisions. 4. Increased collisions enhance diffusion rate. Summary of incorrect choices: B. Space between particles increasing doesn't directly affect diffusion rate. C. Density of particles decreasing doesn't necessarily impact diffusion rate. D. Size of particles increasing doesn't affect diffusion rate directly.

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: C

Rationale: To calculate the molar mass of hydrogen iodide (HI), we add the atomic masses of hydrogen (1 g/mol) and iodine (127 g/mol). Therefore, the molar mass of HI is 1 + 127 = 128 g/mol. Rounding to the nearest whole number, the mass of one mole of hydrogen iodide is 128 g/mol, making choice C (87 g/mol) incorrect. Choices A (2 g/mol) and B (58 g/mol) are also incorrect as they do not reflect the correct molar mass of hydrogen iodide.

Correct Answer: B

Rationale: The correct answer is B: Pyridine (C5H5N) because it is not a hydrocarbon. Hydrocarbons are compounds composed of only carbon and hydrogen atoms. Pyridine contains nitrogen in addition to carbon and hydrogen, making it a heterocyclic compound. Methane, ethane, and propane are all hydrocarbons as they only contain carbon and hydrogen atoms. Pyridine is not a hydrocarbon as it contains nitrogen in its chemical structure, making it the only non-hydrocarbon option among the choices.

Correct Answer: A

Rationale: The correct answer is A: H⁻. A hydride ion is formed when a hydrogen atom gains an extra electron, resulting in a negatively charged ion. The notation for a hydride ion is H⁻, indicating the presence of a single hydrogen atom with a negative charge. This is the only choice that accurately represents a hydride ion. Explanation of incorrect choices: B: H⁺ - This represents a hydrogen ion with a positive charge, not a hydride ion. C: OH⁻ - This represents a hydroxide ion, not a hydride ion. D: HCl - This is a compound consisting of a hydrogen atom and a chlorine atom, not a hydride ion.