  In this work we investigate the thermodynamic properties satisfied by an
expanding universe filled with a monoatomic ideal gas. We show that the
equations for the energy density, entropy density and chemical potential remain
the same of an ideal gas confined to a constant volume V . In particular the
Sackur-Tetrode equation for the entropy of the ideal gas is also valid in the
case of an expanding universe, provided that the constant value that represents
the current entropy of the universe is appropriately chosen.
