## deltaS question

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

Jessica Jones 2B
Posts: 80
Joined: Fri Sep 29, 2017 7:04 am

### deltaS question

Hi! In the solutions manual I have seen them go back and forth between using deltaS=nCpln(t2/t1) and deltaS=nRln(t2/t1). I am confused on if we are supposed to be using Cp or R? or base it on the problem. Do you only use Cp when doing the (3/2*R) or (5/2*R)?

Masih Tazhibi 2I
Posts: 33
Joined: Fri Sep 29, 2017 7:06 am

### Re: deltaS question

The 3/2 and 5/2 is for ideal gases that have different properties. For gases that are not ideal, you use the one without Cp.

Sabah Islam 1G
Posts: 50
Joined: Sat Jul 22, 2017 3:01 am

### Re: deltaS question

The Cp (5/2R) and Cv (3/2R) are the specific heat capacities for ideal gases only. When an ideal gas is not involved in a problem, you would use the value of R.

Anna Li 2E
Posts: 21
Joined: Sat Jul 22, 2017 3:00 am

### Re: deltaS question

Also when solving equations that involve ideal gases, Cp is used for constant pressure and Cv is used for constant volume. Usually it is specified in the problem whether the ideal gas is under constant pressure/volume so substitute Cp or Cv accordingly.

McKenna disc 1C
Posts: 60
Joined: Fri Sep 29, 2017 7:04 am

### Re: deltaS question

The consensus appears to be that we use R for non-ideal gases and Cp or Cv for ideal gases. Should we expect to be told whether or not to assume ideal behavior for the gases in the problems?
Thanks!

Posts: 88
Joined: Fri Sep 29, 2017 7:03 am

### Re: deltaS question

Yes, I believe most of the time, the questions will tell you whether or not it's an ideal gas.

Dang Lam
Posts: 55
Joined: Thu Jul 27, 2017 3:01 am

### Re: deltaS question

Yes, Cp is use for ideal gas only!!!

aaron tang 2K
Posts: 49
Joined: Thu Jul 27, 2017 3:01 am

### Re: deltaS question

5/2 and 3/2 are used for specific heat capacities of ideal gases. For the problems in the book, you should only use Cp when dealing with ideal gases, if not, then use R for non-ideal gases