SAT Subject Physics Practice Question 289: Answer and Explanation

Next steps

Question: 289

60. If a container contains a mixture of two ideal gases (of different molecular masses) at thermal equilibrium, which of the following is true?

A. The average kinetic energy of the molecules of the lighter gas is less than the average kinetic energy of the molecules of the heavier gas.
B. The average kinetic energy of the molecules of the lighter gas is greater than the average kinetic energy of the molecules of the heavier gas.
C. The average speed of the molecules of the lighter gas is less than the average speed of the molecules of the heavier gas.
D. The average speed of the molecules of the lighter gas is equal to the average speed of the molecules of the heavier gas.
E. The average speed of the molecules of the lighter gas is greater than the average speed of the molecules of the heavier gas.

Correct Answer: E

Explanation:

Since the gases are in thermal equilibrium in the same container, they’re at the same temperature, and because the average kinetic energy of the molecules is proportional to the temperature, the fact that their temperatures are the same implies that the average kinetic energy of their molecules is the same also. This eliminates A and B. Now, in order for the lighter molecules to have the same average kinetic energy as the heavier ones, the lighter molecules must be moving faster on average, so E is correct.

Previous       Next