Chemistry Hesi A2

Chemistry Hesi A2 Related

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A chemist takes 100 mL of a 40 g NaCl solution and dilutes it to 1L. What is the concentration (molarity) of the new solution?

  • A. 0.04 M NaCl
  • B. 0.25 M NaCl
  • C. 0.40 M NaCl
  • D. 2.5 M NaCl
Correct Answer: C

Rationale: Initially, the chemist has 40 g of NaCl in 100 mL of solution. To find the initial molarity, we need to calculate the number of moles of NaCl using the molar mass of NaCl (58.44 g/mol). After dilution to 1 L, the molarity of the new solution can be calculated by dividing the moles of NaCl by the total volume in liters. Therefore, the concentration (molarity) of the new solution is 0.40 M NaCl. Choice A (0.04 M NaCl) is incorrect because it doesn't consider the correct molar concentration after dilution. Choice B (0.25 M NaCl) is incorrect as it also doesn't account for the correct molar concentration post-dilution. Choice D (2.5 M NaCl) is incorrect as it is too concentrated given the initial amount of NaCl and the dilution factor.

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The molar mass of some gases is as follows: carbon monoxide-28.01 g/mol; helium-4.00 g/mol; nitrogen-28.01 g/mol; and oxygen-32.00 g/mol. Which would you expect to diffuse most rapidly?

  • A. Carbon monoxide
  • B. Helium
  • C. Nitrogen
  • D. Oxygen
Correct Answer: B

Rationale: The rate of diffusion is inversely proportional to the molar mass of the gas. Helium has the lowest molar mass among the given gases, making it the lightest and fastest gas to diffuse. Therefore, helium would be expected to diffuse most rapidly compared to carbon monoxide, nitrogen, and oxygen. Carbon monoxide, nitrogen, and oxygen have higher molar masses than helium, so they would diffuse more slowly. Therefore, the correct answer is helium.

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.

Which is a triatomic allotrope of oxygen?

  • A. Ozone
  • B. Water
  • C. Acidic oxide
  • D. Carbon dioxide
Correct Answer: A

Rationale: Ozone (O3) is a triatomic allotrope of oxygen. It differs from the common diatomic oxygen molecule (O2) by having three oxygen atoms bonded together. Ozone is known for its protective role in the Earth's atmosphere, absorbing most of the Sun's harmful ultraviolet radiation. Water (H2O) is a compound composed of two hydrogen atoms and one oxygen atom. Acidic oxide and carbon dioxide are not triatomic allotropes of oxygen. Carbon dioxide consists of one carbon atom and two oxygen atoms, while acidic oxides refer to compounds where oxygen is bonded with other elements to form oxides, and they are not allotropes of oxygen.

Which compound has a nonpolar bond in which the electrons are shared equally?

  • A. Hâ‚‚O
  • B. NH₃
  • C. Clâ‚‚
  • D. CHâ‚„
Correct Answer: D

Rationale: The compound CH₄, methane, has a nonpolar bond where carbon and hydrogen share electrons equally. This occurs because carbon and hydrogen have similar electronegativities, meaning they have equal abilities to attract shared electrons. Consequently, a nonpolar covalent bond is formed due to the balanced sharing of electrons between these atoms. Choices A, B, and C do not have nonpolar bonds with electrons shared equally. In H₂O (water), there are polar covalent bonds due to the difference in electronegativity between hydrogen and oxygen. In NH₃ (ammonia), the nitrogen-hydrogen bonds are polar because of the electronegativity difference. In Cl₂ (chlorine gas), the Cl-Cl bond is nonpolar, but the question specifies a compound, not an element, and chlorine does not share its electrons equally with another element in a compound.

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.

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