In this paper we find a full connection between the long lasting macroscopic classical laws of gases and the quantum mechanical description of non-interacting particles confined in a box, thus constituting an ideal gas. In such a gas, the motion of each individual molecule can be considered to be independent of all other molecules, and the macroscopic parameters of an ideal gas, mainly, pressure P and temperature T, can be defined as simple average quantities based on individual motions of all molecules in consideration. It is shown that for an ideal gas enclosed in a macroscopic cubic box of volume V, an alphanumeric expression for the Constant appearing in the classical law of adiabatic expansion law, i.e. PV5/3 = Constant, can be derived based on quantum mechanics. Note that this constant has otherwise remained for centuries, as just an abstract quantity in the form of P1V15/3=P2V25/3 = P3V35/3 written for different thermodynamic states, delineated through an adiabatic transformation. No one even seems to have thought that it may eventually have a particular expression. Physical implications of the result we disclose are discussed.