The root mean square speed, crms, can be related to macroscopic properties;

By considering the rate of change of the momentum of particles colliding elastically with a wall, we can find the equation: pV = nM(crms)2/3 where

n = moles of gas
M = molecular mass in Kgmol-1
p = pressure
V = volume of the container

And since we know for a perfect gas pV = nRT, we have RT = M(crms)2/3, where

R = universal gas constant
T = temperature in K