HESI A2 Chemistry

Correct Answer: B

Rationale: The correct formula for potassium chloride is KCl. "K" represents potassium and "Cl" represents chloride. Potassium is a metal with a +1 charge, while chloride is a non-metal with a -1 charge. When these two ions combine, they form a neutral compound with a 1:1 ratio, resulting in the formula KCl. Choice A (NaCl) is incorrect because "Na" represents sodium, not potassium. Choice C (KCl) is also incorrect because it is the same as the correct answer. Choice D (ClK) is incorrect as the order of elements in a chemical formula matters, and in this case, it should be KCl to represent potassium chloride accurately.

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 (-1) because in NaOH, Na is a group 1 metal with an oxidation state of +1, and H is typically +1. Since the compound is neutral, the oxidation state of oxygen must be -1 to balance the charges. Choice A (-2) is incorrect because oxygen rarely has an oxidation state of -2 in compounds. Choice C (0) is incorrect as oxygen is not in its elemental form. Choice D (+2) is incorrect as it is not a common oxidation state for oxygen in compounds.

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.

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.