HESI A2
Chemistry Hesi A2
1. A radioactive isotope has a half-life of 20 years. How many grams of a 6-gram sample will remain after 40 years?
- A. 8
- B. 6
- C. 3
- D. 1.5
Correct answer: C
Rationale: The half-life of a radioactive isotope is the time it takes for half of the original sample to decay. After each half-life period, half of the initial sample remains. In this case, after the first 20 years, half of the 6-gram sample (3 grams) will remain. After another 20 years (total of 40 years), half of the remaining 3 grams will remain, which is 1.5 grams. Therefore, 3 grams will be left after 40 years. Choice A is incorrect as it doesn't consider the concept of half-life and incorrectly suggests an increase in the sample. Choice B is incorrect as it assumes no decay over time. Choice D is incorrect as it miscalculates the remaining amount after two half-life periods.
2. How many electrons are in a neutral atom of neon?
- A. 9
- B. 10
- C. 11
- D. 12
Correct answer: B
Rationale: The atomic number of neon is 10, which represents the number of protons in its nucleus. In a neutral atom, the number of electrons is equal to the number of protons to maintain electrical neutrality. Therefore, a neutral atom of neon contains 10 electrons, matching the 10 protons within the nucleus. Choice A (9 electrons) is incorrect as it doesn't correspond to the atomic number of neon. Choices C (11 electrons) and D (12 electrons) are also incorrect as they do not align with the correct atomic number of neon.
3. What is the correct formula for sodium nitrate?
- A. NaNO
- B. Na NO
- C. NaNO₃
- D. Na NO₂
Correct answer: C
Rationale: The correct formula for sodium nitrate is NaNO₃. In this formula, 'Na' represents sodium, 'N' represents nitrogen, and 'O₃' represents three oxygen atoms. Sodium nitrate consists of one sodium ion (Na⁺) and one nitrate ion (NO₃⁻), which means the correct formula is NaNO₃. Choice A (NaNO) is incorrect as it lacks the subscript indicating the presence of three oxygen atoms. Choice B (Na NO) is incorrect as it includes a space between 'Na' and 'NO', which is not part of the standard chemical formula notation. Choice D (Na NO₂) is incorrect as it indicates a different compound with a nitrite ion (NO₂⁻) instead of nitrate ion.
4. Which of these intermolecular forces might represent attraction between atoms of a noble gas?
- A. Dipole-dipole interaction
- B. London dispersion force
- C. Keesom interaction
- D. Hydrogen bonding
Correct answer: B
Rationale: Noble gases are non-polar molecules without a permanent dipole moment. The only intermolecular force applicable to noble gases is the London dispersion force, also known as Van der Waals forces. This force is a temporary attractive force resulting from the formation of temporary dipoles in non-polar molecules. Dipole-dipole interactions, Keesom interactions, and hydrogen bonding involve significant dipoles or hydrogen atoms bonded to electronegative atoms, which do not apply to noble gases.
5. What is the charge of a gamma ray?
- A. -1
- B. +1
- C. +2
- D. No charge
Correct answer: D
Rationale: Gamma rays are a form of electromagnetic radiation with no charge. They are neutral particles that do not possess any electric charge. This characteristic allows them to be unaffected by electric or magnetic fields. Additionally, gamma rays travel at the speed of light in a vacuum. Choices A, B, and C are incorrect as gamma rays do not carry a charge of -1, +1, or +2; they are neutral entities.
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