The calculation of entropy for chemical systems

3 The calculation of entropy for chemical systems In the Clausius approach to entropy, one gets  

For areal gas or for a condensed phase, (9 V/9T)p is the thermal expansion coefficient of the material at constant pressure, a measurable quantity, so entropy variations with pressure can be easily calculated. (dV/9T)p is very small for condensed phases, and entropy is hardly sensitive to pressure variations in liquids and solids. In any case, the standard entropy is defined as the entropy at unit pressure (usually 1 atm, but sometimes also 1 bar). For a reversible process in which only F-V work is exchanged. 

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