HESI A2 Chemistry Practice Questions

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: 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: B

Rationale: The correct answer is B (H₂SO₄) because it is a strong acid that completely dissociates into ions in water, leading to a high concentration of H⁺ ions. This is indicated by its ability to donate multiple protons. Choice A (CH₃COOH) is a weak acid that only partially dissociates. Choice C (NH₃) is a weak base that accepts protons rather than donating them. Choice D (KOH) is a strong base that dissociates completely in water but is not an acid.

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: 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