Entropy in Ideal Gas State

Entropy in Ideal Gas State.—The computation of the entropy of an organic compound in the gas phase from its entropy in a condensed phase was referred to in the introduction. The additional experimental data required for the computation are sometimes available from vapour-pressure measurements and from vapour-flow calorimetry. For example, the entropy of diethyl ether in the ideal gas state (298.15 K, 101.325 kPa) 

Afly/rfrom measurements of Affy by vapour-flow calorimetry 

Si — St = pdBJdT, derived from vapour heat capacities measured by vapour-flow calorimetry 

If heat capacities of the compound are available at temperatures higher than 298 K, it may be advantageous to calculate 5 (g, 298.15 K) from the entropy of the condensed phase at a higher temperature. Errors arising from extrapolation of enthalpies of vaporization and vapour pressures may thus be reduced. For example, the entropy of propan-l-ol gas was calculated as follows  

350 K) from measurements AHyjTat 350 K from vapour-flow calorimetry 7 In p at 350 K from vapour pressure measurements 

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