Statistical Mechanics/Ideal Gas

Consider an ensemble of molecules.

 Take  a  paticular  molecule,  while the others form  a  heat reservoir  at  temperature  T.
The energy(E)is kinetic = p2/2m = 1/2mv2,

P(E) α e^-βE α e^-β(p2/2m)

The probability that the molecule lies between r^→ and r^→+