Which of the following gases has the highest average speed at 400K?

A. NO_2
B. O_2
C. F_2
D. Cl_2
According to my answer sheet, the correct answer is A, but I don't understand why. From my understanding, shouldn't the gas with the lowest molar mass have the highest speed?

1 Answer
Dec 17, 2017

B. O_2 should be the correct answer for the reasoning given.

Explanation:

Just as you stated, the gas with the lowest molar mass should have the highest speed, and O_2 has the lowest molar mass. This relationship is true since KE = 1/2mv^2, so if the temperature, also known as the average kinetic energy, is constant, mass is inversely related to velocity.