Which of the following gases has the highest average molecular velocity at 25 °C? H2O

He

HCl

BrF

NO2

He

Step-by-step explanation:

A postulate of the Kinetic Molecular Theory is that at a given temperature, the average kinetic energy of the molecules is directly proportional to the Kelvin temperature.

KE = ½ mv² ∝ T

mv² ∝ T Divide each side by T

v² ∝ T/m

If T is constant.

v² ∝ 1/m Take the square root of each side

v ∝ 1/√m

This is an inverse relationship, so the molecules with the smallest molecular mass should have the highest average speeds.

A helium molecule has the smallest mass, so helium has the highest average molecular velocity.