HESI A2 Chemistry Practice Questions

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: 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: A

Rationale: Step-by-step rationale: 1. Heating increases kinetic energy of particles. 2. Higher kinetic energy results in faster movement. 3. Faster movement leads to increased collisions. 4. Increased collisions enhance diffusion rate. Summary of incorrect choices: B. Space between particles increasing doesn't directly affect diffusion rate. C. Density of particles decreasing doesn't necessarily impact diffusion rate. D. Size of particles increasing doesn't affect diffusion rate directly.

Correct Answer: C

Rationale: Failed to generate a rationale after 5 retries.

Correct Answer: C

Rationale: To calculate the molar mass of hydrogen iodide (HI), we add the atomic masses of hydrogen (1 g/mol) and iodine (127 g/mol). Therefore, the molar mass of HI is 1 + 127 = 128 g/mol. Rounding to the nearest whole number, the mass of one mole of hydrogen iodide is 128 g/mol, making choice C (87 g/mol) incorrect. Choices A (2 g/mol) and B (58 g/mol) are also incorrect as they do not reflect the correct molar mass of hydrogen iodide.