Which of the following gases will have the highest velocity at a given temperature?

Ne

O2

He

Cl2

The velocity of gaseous molecule is dependent on the molecular weight at constant temperature. In this case, Ne has a weight of 20.2 g/mol, O2 has 32 g/mol, He has 4 g/mol, Cl2 has 70.9 g/mol. The lower the molecular weight, the higher the velocity. Among the choices, helium has the highest velocity.