MCAT General Chemistry Question 113: Answer and Explanation

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Question: 113

8. The plots of two gases at STP are shown below. One of the gases is 1.0 L of helium, and the other is 1.0 L of bromine. Which plot corresponds to each gas and why?

  • A. Curve A is helium and curve B is bromine because helium has a smaller molar mass than bromine.
  • B. Curve A is helium and curve B is bromine because the average kinetic energy of bromine is greater than the average kinetic energy of helium.
  • C. Curve A is bromine and curve B is helium because helium has a smaller molar mass than bromine.
  • D. Curve A is bromine and curve B is helium because the average kinetic energy of bromine is greater than the average kinetic energy of helium.

Correct Answer: C

Explanation:

At STP, the difference between the distribution of speeds for helium and bromine gas is due to the difference in molar mass. Helium has a smaller molar mass than bromine. Particles with small masses travel faster than those with large masses, so the helium gas corresponds to curve B, which has a higher average speed. Because the gases are at the same temperature (273 K), they have the same average kinetic energy, eliminating choices (B) and (D).

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