Question

A container contains a mixture of two ideal gases of different molecular masses at thermal equilibrium, which of the following is true for this mixture?

• 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

Answer 1

The correct option is E The average speed of the molecules of the lighter gas is greater than the average speed of the molecules of the heavier gas

The average speed of gas molecules is $\sqrt{\frac{8RT}{\pi M_0}}$

where $M_0$ is the molecular weight of the gas.
Thus the lighter gas has greater average speed.

However the average kinetic energy is $\frac{3}{2}kT$ which is same for both the gases(thermal equilibrium).