## Winter 2013 Midterm Q1B

Volume: $\Delta S = nR\ln \frac{V_{2}}{V_{1}}$
Temperature: $\Delta S = nC\ln \frac{T_{2}}{T_{1}}$

samuelkharpatin2b
Posts: 55
Joined: Wed Sep 21, 2016 2:57 pm

### Winter 2013 Midterm Q1B

I am wondering why for this problem we use Cv instead of Cp, (shown in step 2 of the solution). I ask because the way I understood it is that the volume increases from 3.0 L to 3.5 L, so I thought the volume isn't constant. Please see attached file for the problem.

Thank you!!
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Rosaline_Chow_2L
Posts: 10
Joined: Fri Jul 22, 2016 3:00 am

### Re: Winter 2013 Midterm Q1B

From what I understand, the entropy equation can only handle one change at a time. So in this case you need to solve for two separate values of entropy and add them together. The first equation is used when temperature is held constant and volume is changed, and the second is where volume is held constant and temperature is changed. Thus, you need to use Cv in the second equation.

samuelkharpatin2b
Posts: 55
Joined: Wed Sep 21, 2016 2:57 pm

Thank you.