1. The problem statement, all variables and given/known data So as a part of a very long question on homework for thermodynamics, I and some of my friends got into a debate about how to write an expression for entropy at a particular state. 2. Relevant equations delS(state 1 to 2)= Cv*ln(T2/T1)+R*ln(V2/V1) (from constitutive relation for entropy change in an ideal gas) 3. The attempt at a solution Is is correct to say S1 (absolute value of entropy of a gaseous system in state 1)= Cv*ln(T1)+R*ln(V1)? I don't think so, coz you can't really calculate the absolute value of entropy ...unless you take the absolute zero temperature as reference state...in which case the above expression would contain -(Cv*ln(T0)+R*ln(V0))...and you'd still be taking 'change' in entropy rather than an 'absolute value.