Which molecule moves with the fastest average speed in the Earth's atmosphere in thermal equilibrium?
A) Water, (atomic mass = 18)
B) Carbon dioxide, (atomic mass = 44)
C) Nitrogen, (atomic mass = 28)
D) Oxygen, (atomic mass = 32)

Question

Which molecule moves with the fastest average speed in the Earth's atmosphere in thermal equilibrium?
A) Water, (atomic mass = 18) 
B) Carbon dioxide, (atomic mass = 44) 
C) Nitrogen, (atomic mass = 28) 
D) Oxygen, (atomic mass = 32)

Quizplus · Accepted Answer

The average speed of a gas molecule is directly proportional to the square root of its temperature and inversely proportional to its molar mass. Since all gases in the Earth's atmosphere are at the same temperature in thermal equilibrium, the fastest moving gas molecule will be the one with the lowest molar mass. Among the choices given, water (H2O) has the lowest molar mass (18 g/mol) and therefore moves with the fastest average speed.

Which Molecule Moves with the Fastest Average Speed in the Earth's