HESI A2
HESI A2 Chemistry
1. What is the boiling point of water in °C?
- A. 90°C
- B. 100°C
- C. 95°C
- D. 80°C
Correct answer: B
Rationale: The correct answer is 100°C. The boiling point of water in Celsius is 100°C, which is standard at sea level. This is the temperature at which water changes from a liquid to a gas phase under standard atmospheric pressure. Choice A (90°C), Choice C (95°C), and Choice D (80°C) are incorrect because they do not represent the standard boiling point of water at sea level.
2. What is atomic mass?
- A. Number of protons in an atom
- B. Sum of protons and neutrons
- C. Number of neutrons in an atom
- D. Average weight of an element
Correct answer: B
Rationale: Atomic mass, also known as atomic weight, is the sum of the number of protons and neutrons in an atom. It represents the average mass of an atom of an element, taking into account the different isotopes and their relative abundance. Neutrons contribute to the atomic mass alongside protons, while the number of neutrons alone is not the definition of atomic mass. Choice A is incorrect because it refers only to the number of protons, not the complete atomic mass. Choice C is incorrect as it focuses solely on the number of neutrons, excluding the contribution of protons. Choice D is incorrect as it mentions the 'average weight of an element,' which is related to atomic mass but does not encapsulate the specific definition of atomic mass as the sum of protons and neutrons.
3. How many neutrons are in an atom of carbon-12?
- A. 2
- B. 4
- C. 6
- D. 24
Correct answer: C
Rationale: An atom of carbon-12 has 6 neutrons. The atomic number of carbon is 6, representing the number of protons. The atomic mass of carbon-12 is 12. To find the number of neutrons, subtract the atomic number from the atomic mass: 12 - 6 = 6 neutrons. Choice A, 2, is incorrect as it does not reflect the correct calculation for neutrons in carbon-12. Choice B, 4, is incorrect as it does not match the correct number of neutrons in carbon-12. Choice D, 24, is incorrect as it is significantly higher than the actual number of neutrons in carbon-12.
4. What is the molarity of a solution containing 45 moles of NaCl in 4 liters?
- A. 0.11 M NaCl
- B. 0.45 M NaCl
- C. 1.8 M NaCl
- D. 8.9 M NaCl
Correct answer: A
Rationale: To calculate the molarity of a solution, you use the formula: Molarity (M) = moles of solute / liters of solution. In this case, M = 45 moles / 4 L = 11.25 M. The correct answer is 0.11 M NaCl. Choice B is incorrect as it doesn't match the calculated value. Choice C is also incorrect as it is significantly higher than the correct molarity. Choice D is incorrect as it is excessively high compared to the calculated value.
5. When balanced, the reaction Fe + O₂ → FeO will be?
- A. 2Fe + 2O₂ → 3FeO
- B. 4Fe + 6O₂ → 6FeO
- C. 2Fe + 3O₂ → 2FeO
- D. 4Fe + 3O₂ → 2FeO
Correct answer: C
Rationale: To balance the chemical equation Fe + O₂ → FeO, the coefficients needed are 2 for Fe and 1 for O. Therefore, the balanced equation becomes 2Fe + O₂ → 2FeO, which translates into 2Fe + 3O₂ → 2FeO. This corresponds to option C. Choice A has the incorrect number of oxygen molecules. Choice B has an incorrect number of Fe atoms on the product side. Choice D also has an incorrect number of Fe atoms on the product side.
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