HESI A2 Chemistry Practice Questions

Correct Answer: A

Rationale: Answer A (NH₃) does not name a polar molecule because it has a trigonal pyramidal shape with a lone pair on nitrogen, causing an uneven distribution of electron density. This results in a net dipole moment, making NH₃ a polar molecule. In contrast, choices B (H₂S), C (SO₂), and D (CO₂) all have symmetrical shapes or polar bonds that cancel out the dipole moments, making them nonpolar molecules.

Correct Answer: B

Rationale: To balance the equation Fe + Cl2 → FeCl3, we need to ensure the same number of each type of atom on both sides. Starting with Fe, we have 1 Fe on the left and 1 Fe on the right, so Fe is balanced. Next, we have 2 Cl on the left from Cl2 and 3 Cl on the right from FeCl3. To balance Cl, we need to multiply Cl2 by 3 to get 6 Cl atoms on each side. Therefore, the correct balanced equation is: 2Fe + 3Cl2 → 2FeCl3. Choice A (2Fe + 2Cl2 → 2FeCl3) is incorrect because it doesn't balance the Cl atoms properly. Choice C (3Fe + 2Cl2 → 3FeCl3) is incorrect because it doesn't balance the Fe atoms. Choice D (3Fe + 3Cl2 → 6FeCl3)

Correct Answer: D

Rationale: The correct answer is D: Lye. Lye, also known as sodium hydroxide, forms hydroxide ions (OH-) when placed in water due to its highly alkaline nature. When lye dissociates in water, it releases hydroxide ions, which contribute to the solution's basicity. Lemon juice (A) and vinegar (C) are acidic solutions that release hydrogen ions (H+) in water. Battery acid (B) typically contains sulfuric acid, which also releases hydrogen ions in water. Therefore, only lye is the substance that forms hydroxide ions when placed in water, making it the correct answer.

Correct Answer: A

Rationale: The correct answer is A: 2. Helium-4 has an atomic number of 2, which represents the number of protons in its nucleus. Since the atomic number is the sum of protons and neutrons, and helium-4 has 2 protons, it must also have 2 neutrons to make the total atomic mass 4. Therefore, there are 2 neutrons in an atom of helium-4. Choice B: 3 is incorrect because it does not match the atomic number of helium-4. Choice C: 4 is incorrect as it is the total atomic mass of helium-4, not the number of neutrons. Choice D: 6 is incorrect as it does not correspond to the atomic structure of helium-4.

Correct Answer: B

Rationale: To find moles of KBr, use formula moles = concentration (M) x volume (L). Given 4 M KBr solution in 25 mL, convert mL to L (25 mL = 0.025 L). Calculate moles = 4 mol/L x 0.025 L = 0.1 mol. Therefore, the correct answer is B (0.1 mol). Option A (0.035 mol) is too low, C (0.18 mol) is too high, and D (1.6 mol) is significantly higher due to incorrect calculations.