  We employ classical thermodynamics to gain information about absolute
entropy, without recourse to statistical methods, quantum mechanics or the
Third Law of thermodynamics. The Gibbs-Duhem equation yields various simple
methods to determine the absolute entropy of a fluid. We also study the entropy
of an ideal gas and the ionization of a plasma in thermal equilibrium. A single
measurement of the degree of ionization can be used to determine an unknown
constant in the entropy equation, and thus determine the absolute entropy of a
gas. It follows from all these examples that the value of entropy at absolute
zero temperature does not need to be assigned by postulate, but can be deduced
empirically.
