Problem: Heat #1

The air in a room consists principally of nitrogen and oxygen. The molecular weight of nitrogen (N₂) is 28 g/mole and the molecular weight of oxygen (O₂) is 32 g/mole. The air is in thermal equilibrium. Which statement is true?

A.	The average speeds of the nitrogen and oxygen molecules are equal.
B.	The average kinetic energies of the nitrogen and oxygen molecules are equal.
C.	The oxygen molecules have average speeds that are greater than for the nitrogen molecules.
D.	The nitrogen molecules have average kinetic energies that are greater than for the oxygen molecules.
E.	None of the above statements are true.

Solution

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The answer is B. When it is said that the air is in thermal equilibrium, all the constituents of the air - including nitrogen and oxygen gases - are having the same temperature. Because temperature is the measure of the average kinetic energy of the atoms and molecules of a substance, the average kinetic energies of the nitrogen and oxygen molecules must be equal.

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Question source: Saskatchewan High School Physics Scholarship Competition 2006