Isothermic change of state means the temperature of a gas mixture is constant, when the pressure
and volume are changed.
q = m x R x T x ln(P2/P1)
q = P1 x V1 x T x ln(V2/V1)
  q  : The quantity of heat ( j )
  m : mass ( Kg )
  V : volume (m3)
  T : absolute temperature ( K )
  R : individual gas constant  (J/Kg x K)
  P : absolute pressure ( Pa )