# Homework Help: Archived Isothermal expansion of gas question

1. Oct 30, 2013

### paulnz

Hello, I am just wondering if it is possible to actually do the following question based on the information given
1. The problem statement, all variables and given/known data
An isothermal reversible expansion, for which pV=constant as shown by the curve

joining State A and State B, takes a system from State A to State B along the curved

line on the diagram above. The pressure of State A is 50 x 10^5 Pa and the pressure of

State B is 12.5 x 10^5 Pa. The system is also taken from State A to State B in two

irreversible steps as indicated by the pathways I and II in the diagram. Each irreversible

step involves expansion against a constant external pressure.

If the system is an ideal gas determine the entropy change for the reversible

expansion from State A to State B, and calculate the increase in the number of

microstates for the system during expansion.

2. Relevant equations
pV=constant

3. The attempt at a solution

I would have thought you would have required the number of moles of the gas to solve the problem, but I could be wrong? That's what is confusing me. Unless the fact that pV= constant has something to do with it?

attached is the graph as the problem requires.

Thanks for any help.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

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2. Feb 7, 2016

### CrazyNinja

The entropy change in an isothermal process is zero. @Chestermiller... Is this correct? I got a doubt because the question is long and such a simple answer COULD be wrong.

3. Feb 7, 2016

### Staff: Mentor

No, not in general. Why would you think this?

4. Feb 7, 2016

### CrazyNinja

Q=0. There is no energy exchanged. How will the entropy change?

5. Feb 7, 2016

### Staff: Mentor

Who says Q=0 for all isothermal processes? What if work is done? Then Q = W.

6. Feb 7, 2016

### CrazyNinja

Wait. Did I say Q=0? Forgive me. I meant ΔT=0.

7. Feb 7, 2016

### Staff: Mentor

Entropy is not just a function of temperature. It is also a function of pressure (or volume).

8. Feb 7, 2016

### CrazyNinja

OK. I guess I have to look up entropy again. By the way, can we do this question using the given information? If yes, then say so, but do not write solution. I will try it out.

9. Feb 7, 2016

### Staff: Mentor

Yes (per mole, unless they tell you the number of moles or the temperature). The OP has not shown the full problem statement, so it not clear whether the temperature was given.

Last edited: Feb 7, 2016