HESI A2 Chemistry Practice Test

Correct Answer: D

Rationale: The correct answer is D, the law of constant composition. This law states that a compound always contains the same elements in the same proportion by mass. In the case of water, the ratio of hydrogen to oxygen by mass is always 1:8, regardless of the source or quantity of water. This is a key principle in chemistry and helps identify and characterize different compounds. A: The law of multiple proportions deals with the ratios of different elements that can combine to form different compounds, not the same compound. B: The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. C: The law of conservation of energy states that energy cannot be created or destroyed, but this is not directly related to the composition of compounds.

Correct Answer: D

Rationale: The correct answer is D: Wavefunction. A wavefunction in quantum mechanics is a mathematical function used to describe the behavior of particles, such as electrons, in wave-particle duality. It gives the probability amplitude of finding a particle at a specific position, time, and sometimes spin. The other choices are incorrect because: A: Wavelength is the distance between two consecutive points of a wave with the same phase. B: Frequency is the number of wave cycles passing a point per unit of time. C: Wavenumber is the spatial frequency of a wave, defined as the number of waves per unit distance.

Correct Answer: C

Rationale: Henry's Law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid (P = kC). This means that as the partial pressure of the gas increases, the amount of gas that dissolves in the liquid also increases. This relationship is specifically for gases and volatile substances in liquids. Boyle's Law relates pressure and volume, Gay-Lussac's Law relates pressure and temperature, and Charles' Law relates volume and temperature. Therefore, the correct answer is C, Henry's Law, because it directly describes the relationship between gas solubility and partial pressure.

Correct Answer: B

Rationale: The correct answer is B: Alloys. Alloys are mixtures of 2 or more metals, where the metals are evenly distributed at a microscopic level. This creates unique properties different from the individual metals. Solutions (A) involve a solute dissolved in a solvent, not necessarily metals. Compounds (C) are formed by chemical bonding between elements, not just a mixture. Suspensions (D) are mixtures where particles are suspended in a fluid but eventually settle out, unlike alloys which are uniform mixtures.

Correct Answer: B

Rationale: The correct answer is B: 20. Essential amino acids are those that the human body cannot produce on its own and must be obtained from diet. Humans require 9 essential amino acids. Since the total number of amino acids required for human life is 20, including the 9 essential ones, the correct answer is 20. Choices A, C, and D are incorrect because they do not align with the established fact that humans need 20 amino acids for survival, with 9 being essential.