HESI A2 Chemistry Practice Questions

Correct Answer: C

Rationale: The correct answer is C: 5 g. After 10 years, half of the cobalt-60 decays, leaving 10 g. After 5 more years, another half will decay, leaving 5 g. This is because the half-life of cobalt-60 is 5 years, meaning it takes 5 years for half of the initial amount to decay. Choices A, B, and D are incorrect because they do not take into account the concept of half-life and the exponential decay of radioactive substances.

Correct Answer: C

Rationale: Rationale for the correct answer (C): A saturated solution has the maximum concentration of solute dissolved in the solvent. At this point, the rate of dissolution equals the rate of crystallization, achieving a dynamic equilibrium. As a result, adding more solute will not increase its concentration further. Choice A is incorrect because a saturated solution has exactly the right amount of solute dissolved. Choice B is incorrect because a saturated solution is at its maximum solubility. Choice D is incorrect because a precipitate would indicate an unsaturated solution.

Correct Answer: D

Rationale: The correct answer is D: Silicon. Silicon has an atomic mass greater than that of sodium (Na). Sodium has an atomic mass of 22.99 g/mol, whereas silicon has an atomic mass of 28.09 g/mol. Therefore, silicon's atomic mass is greater than that of sodium. Boron (A), oxygen (B), and fluorine (C) all have atomic masses lower than sodium, making them incorrect choices in this context.

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.

Correct Answer: B

Rationale: The correct answer is B: London dispersion force. This is the weakest intermolecular force because it occurs between temporary dipoles induced by momentary fluctuations in electron distribution. It is present in all molecules, regardless of polarity. Dipole-dipole interactions (A) are stronger as they involve permanent dipoles, hydrogen bonding (C) is stronger and specific to molecules with hydrogen directly bonded to highly electronegative atoms, and ionic bonding (D) is the strongest intermolecular force due to the electrostatic attraction between oppositely charged ions.