So = S(liquid or crystal) + (Hvap or Hvap)/T + S* + R×T×ln(Vo/Vsat), where
T is temperature (K) and po is the standard pressure (100 kPa)
S(liquid or crystal) is the entropy of the condensed phase at temperature T (obtained by integration of heat capacity)
Hvap or Hvap is the enthalpy of vaporization or sublimation at temperature T
S* is the non-ideality correction for the vapor density at the saturated vapor pressure at temperature T:
S* = So - S(real gas) = R×((B + T×dB/dT)/V + (C - T×dC/dT)/2V2 - ln(Z))
where R is the gas constant, Z is the compressibility factor, B and C are the 2nd and 3rd virial coefficients. [Properties of Gases and Liquids, Table 6-2, Page 6-8]
Vsat and Vo are the ideal-gas volumes at the saturated vapor pressure and po, respectively.