# Homework Help: Chemical thermodynamics, entropy, Maxwell relation and Z

1. Oct 21, 2016

### deeko1987

1. The problem statement, all variables and given/known data

A gas obeys the equation of state

z=1/(1-b(molar density))

What is the molar entropy change of expanding this gas from a volume of 1 dm3 to 2 dm3 at a temperature of 300 K, given b = 0.2 dm3? Calculate your answer to three significant figures and select one answer from the list below.

You may wish to use the Maxwell relation

(∂p/∂t) = ∂s/∂v LHS constant V RHS constant T

2. Relevant equations

PV=RT maxwell realation possibly z = PV/RT

how can I solve for the molar density to solve the compressibility factor. this is an example exam question so I already know the answer is 6.74 J

3. The attempt at a solution

I have substituted the LHS of the maxwell relationship for R/T (not sure if this is a correct assuption) then intergrated to get Rln(V2/V1)= change in entropy

this gives me 5.76 I'm assuming that this then needs multiplying by Z to get the correct answer of 6.74.

I have attempted to solve for z using the 1/volume to get molar density using both densities which give the incorrect answer. I have then tried to average the Z of both (Z1+Z2)/2 which again was a long shot, it was close but I'm assuming that is not the way its done because it seems very inaccurate to use an average for compressibility.

Again this is not a coursework it is a multiple choice practice exam question, that gives the solution, just no feedback.

2. Oct 22, 2016

anyone?