# Partion Function

1. Jul 2, 2009

### senan

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

One mole of a gas obeys the equation of state
(P +a/V2 )(1 −b/V) =RT/V
Find the change in entropy if one mole of the gas is isothermally expanded until the volume
is doubled. Express your answer in terms of the initial volume of the gas Vi.

2. Relevant equations

S2-S1=1/T Integral from Vi to 2Vi of PdV

3. The attempt at a solution

I replaced P with P=NkT/V

S2-S1=-kln(2Vi/Vi)

as the answer was asked for in terms of Vi I'm not sure thats right

sorry titled the question wrong meant to title other Q as partition function

Last edited: Jul 2, 2009
2. Jul 2, 2009

### G01

That is not right.

You are using the wrong equation of state.

It is stated in problem that the gas obeys a different equation of state, not the usual, PV=NkT.

You have to use the given equation of state to express P in terms of V.

3. Jul 4, 2009

### ideasrule

By the way, that equation of state is called the van der Waals equation. It takes into account intermolecular forces and the volume of molecules instead of assuming an ideal gas.