Chemistry HESI A2 Practice Test

Correct Answer: C

Rationale: The given chemical equation shows aluminum replacing hydrogen in sulfuric acid, which is a single replacement reaction. In this type of reaction, one element displaces another in a compound. Here, Al replaces H in H2SO4 to form Al2(SO4)3 and HNO3. The other choices are incorrect because a decomposition reaction involves a single compound breaking down into simpler substances, a synthesis reaction involves two or more substances combining to form a new compound, and a double replacement reaction involves the exchange of ions between two compounds.

Correct Answer: C

Rationale: The correct answer is C: Number of neutrons. Isotopes have the same number of protons (same atomic number), but different numbers of neutrons (different mass numbers). Carbon-12 and carbon-14 both have 6 protons (atomic number for carbon), but carbon-12 has 6 neutrons while carbon-14 has 8 neutrons. Therefore, the common trait between isotopes is the number of neutrons. Explanation for why other choices are incorrect: A: Number of nuclear particles - Isotopes have different numbers of neutrons, so they do not have the same number of nuclear particles. B: Number of protons - Isotopes have the same number of protons, but different numbers of neutrons. D: Mass number - Isotopes have different mass numbers due to differing numbers of neutrons, so this is not the common trait between isotopes.

Correct Answer: B

Rationale: The correct answer is B: Anion. When an atom gains one or more electrons, it becomes negatively charged, forming an anion. This process is known as gaining electron(s) to achieve a stable electron configuration. A cation (A) is formed when an atom loses electron(s) and becomes positively charged. Isotope (C) refers to atoms of the same element with different numbers of neutrons. Electron (D) is a fundamental subatomic particle, not a term for an atom gaining electrons.

Correct Answer: B

Rationale: The molar mass of a substance is the mass of one mole of that substance. The molar mass of water (H2O) can be calculated by adding the atomic masses of its components (2 hydrogen atoms + 1 oxygen atom). Hydrogen's atomic mass is approximately 1 g/mol, and oxygen's atomic mass is approximately 16 g/mol. Therefore, the molar mass of water is 2(1) + 16 = 18 g/mol. This makes choice B (18 g/mol) the correct answer. Choices A, C, and D are incorrect because they do not reflect the accurate molar mass calculation of water based on the atomic masses of its components.

Correct Answer: C

Rationale: The correct answer is C: +2. An alpha particle consists of two protons and two neutrons, giving it a total charge of +2. Protons have a charge of +1 each, and neutrons have no charge. Therefore, the total charge of an alpha particle is +2. Choice A (-1) is incorrect because it is the charge of an electron. Choice B (+1) is incorrect because it represents the charge of a proton. Choice D (No charge) is incorrect because alpha particles have a positive charge due to the presence of protons.