Why is the pressure of a gas does not depend on the molar mass of the gas?
The pressure depends on the average (1/2)mv^2 of the gas molecules
but it is (1/2) m v^2 that you get from kinetic theory and not (1/2) v^2
the gas has the same kinetic energy at temperature T no matter what its mass might be.
see:
http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/kintem.html#c1