HESI A2 Chemistry Practice Questions

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: 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.

Correct Answer: C

Rationale: The correct formula for iron III oxide is Fe₂O₃. In this compound, iron has a +3 oxidation state and oxygen has a -2 oxidation state. By applying the criss-cross method to balance the charges, we get Fe₂O₃. Choice A (IO) is incorrect as it represents a combination of iodine and oxygen. Choice B (FeS) is incorrect as it represents iron and sulfur. Choice D (OFe₂₃) is incorrect as it has the elements in the wrong order and incorrect subscripts. Fe₂O₃ is the correct formula based on the charges and combining the elements in the correct ratios.

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: D

Rationale: The molarity of a solution is defined as the number of moles of solute per liter of solution. This is because molarity is a measure of concentration and represents the ratio of moles of solute to liters of solution. In this case, the molarity of an aqueous sugar solution would be best defined as moles of sugar per liter of solution (Choice D). Explanation: A: Grams of sugar per milliliter of solution - This choice does not consider the volume of the solution and is not a standard unit for molarity. B: Moles of sugar per milliliter of solution - Molarity is defined per liter of solution, not per milliliter. C: Grams of sugar per liter of solution - Molarity is based on moles, not grams, and it is per liter of solution, not per milliliter. Summary: Choice A and B do not account for the volume of the solution, and Choice C uses the incorrect unit