HESI A2 Chemistry

Correct Answer: C

Rationale: To determine the time for the substance to decay to 7 kg, we need to calculate the number of half-lives required. Each half-life reduces the substance by half. Given: Initial mass = 56 kg, Final mass = 7 kg, Half-life = 12 days. Number of half-lives = log(final mass / initial mass) / log(0.5) = log(7/56) / log(0.5) = 3.807 / -0.301 = -12.66. As the number of half-lives must be a whole number, we round up to 13 half-lives. Total time = 13 * 12 days = 156 days. Therefore, the correct answer is C (36 days). The other choices are incorrect because they do not accurately represent the calculation based on the half-life of the substance.

Correct Answer: C

Rationale: The correct answer is C: black. Red and white phosphorus are well-known allotropes. Black phosphorus is another important allotrope with a layered structure. It is stable at room temperature and has unique electronic properties. Green, gray, and silver are not commonly recognized allotropic forms of phosphorus, making them incorrect choices.

Correct Answer: C

Rationale: The correct answer is C: 6. In an atom of carbon-12, the atomic number (number of protons) is 6. Since the atomic number represents the number of protons, to find the number of neutrons, we subtract the atomic number from the mass number. Carbon-12 has a mass number of 12, so 12 - 6 = 6 neutrons. Other choices are incorrect because: A: 2 - This is not correct because the number of neutrons is not equal to the atomic number in carbon-12. B: 4 - This is incorrect as it does not consider the correct calculation based on the atomic number and mass number. D: 24 - This answer is incorrect as it is not related to the atomic structure of carbon-12.

Correct Answer: B

Rationale: The correct answer is B: Butane. In this compound, CH₃-CH₂-CH₂-CH₃, there are four carbon atoms connected in a straight chain, making it a butane. The prefix "but-" indicates that there are four carbon atoms, and the suffix "-ane" indicates that the compound contains only single bonds between the carbon atoms. Therefore, the correct name is Butane. Choice A: Cyclobutane is incorrect because it would be a cyclic compound with four carbon atoms forming a ring. Choice C: Butene is incorrect because it would have a double bond between two carbon atoms, making it an alkene. Choice D: Butyne is incorrect because it would have a triple bond between two carbon atoms, making it an alkyne.

Correct Answer: C

Rationale: The correct answer is C: Cl and P. Both chlorine (Cl) and phosphorus (P) are in the same group on the periodic table (Group 7A/17) and therefore have similar outer electron configurations. This results in similar reactivity patterns, such as forming covalent compounds. Choices A, B, and D are incorrect because they involve elements from different groups with different electron configurations and reactivity patterns. He (helium) and H (hydrogen) are both in different groups and have very different properties. K (potassium) and Ar (argon) are from different groups and have different reactivity levels. Ba (barium) and Mg (magnesium) are also from different groups and exhibit different reactivity behaviors.