HESI A2
HESI A2 Chemistry Practice Questions
1. How many neutrons are in an atom of helium-4?
- A. 2
- B. 3
- C. 4
- D. 6
Correct answer: A
Rationale: The atomic number of helium is 2, indicating it has 2 protons. Helium-4, the most common isotope of helium, has 2 neutrons in addition to its 2 protons. Therefore, the correct answer is 2 neutrons in an atom of helium-4. Choice B, C, and D are incorrect as they do not match the correct composition of helium-4, which consists of 2 protons and 2 neutrons.
2. How many moles of potassium bromide are in 25 mL of a 4 M KBr solution?
- A. 0.035 mol
- B. 0.1 mol
- C. 0.18 mol
- D. 1.6 mol
Correct answer: B
Rationale: To find the moles of potassium bromide in 25 mL of a 4 M KBr solution, we first need to convert the volume from milliliters to liters. 25 mL is equal to 0.025 L. Then, we use the formula moles = molarity x volume in liters. Substituting the values, moles = 4 M x 0.025 L = 0.1 mol. Therefore, there are 0.1 moles of KBr in 25 mL of a 4 M solution. Choice A, 0.035 mol, is incorrect as it does not properly calculate the moles. Choice C, 0.18 mol, and choice D, 1.6 mol, are also incorrect as they are not the result of the correct calculation based on the given molarity and volume.
3. Which element has an atomic mass greater than that of sodium?
- A. Boron
- B. Oxygen
- C. Fluorine
- D. Silicon
Correct answer: D
Rationale: Silicon has an atomic mass greater than that of sodium. The atomic mass of silicon is approximately 28.0855 u, whereas the atomic mass of sodium is approximately 22.9898 u. Therefore, silicon has a greater atomic mass compared to sodium. Boron, Oxygen, and Fluorine have atomic masses lower than sodium, making them incorrect choices in this context.
4. Balance this equation: Fe + Cl2 → FeCl3
- A. 2Fe + 2Cl2 → 2FeCl3
- B. 2Fe + 3Cl2 → 2FeCl3
- C. 3Fe + 2Cl2 → 3FeCl3
- D. 3Fe + 3Cl2 → 6FeCl3
Correct answer: B
Rationale: In the given equation, Fe combines with Cl to form FeCl3. To balance the equation, we need to have the same number of each element on both sides. Since Cl is represented as Cl2 in the equation, we need 3 Cl2 molecules to balance Fe, resulting in 2Fe + 3Cl2 → 2FeCl3. Choice A is incorrect because it only balances Fe but not Cl2. Choice C is incorrect as it balances Fe but not Cl2. Choice D is incorrect as it balances Fe but overbalances Cl2.
5. Which best defines the molarity of an aqueous sugar solution?
- A. Grams of sugar per milliliter of solution
- B. Moles of sugar per milliliter of solution
- C. Grams of sugar per liter of solution
- D. Moles of sugar per liter of solution
Correct answer: D
Rationale: The molarity of a solution is defined as the number of moles of solute per liter of solvent. In the case of an aqueous sugar solution, the molarity would be expressed as moles of sugar per liter of solution. This is because molarity is a measurement of the concentration of a solute in a solution based on the number of moles present in a given volume of the solution. Therefore, the correct answer is D. Choices A, B, and C are incorrect because the molarity is specifically defined in terms of moles of solute per liter of solution, not in grams per milliliter or grams per liter. Molarity is a unit of concentration that relates the amount of solute to the volume of the solution, not the mass of the solute.
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