1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Entropy change in gas

  1. Jan 14, 2015 #1
    Problem statement:
    A sample of 8.02 × 10-1 moles of nitrogen gas ( γ = 1.40) occupies a volume of 2.00 × 10-2 m3at a pressure of 1.00 × 105 Pa and temperature of 300 K. It is isothermally compressed to half its original volume. It behaves like an ideal gas. Find the change in entropy of the gas.

    Relevant equations:

    S2-S1 = Cv loge (P2 / P1) + Cp loge (V2 / V1)

    S2-S1 = Cv loge ((P2V2γ) / (P1V1γ))


    Attempt at answer:
    V2 = 1.00 x 10-2 m3

    P2 can be found by P = nRT / V = 2.00 x 105 Pa

    I assume Cv must be worked out from γ somehow, but I cannot see how to do this.

    Thanks in advance for your help guys!
  2. jcsd
  3. Jan 14, 2015 #2
    How are Cp and Cv related to R (the ideal gas constant)?

  4. Jan 14, 2015 #3
    Cp=Cv+nR surely Cp is needed in order to find Cv?

    Im not even 100% certain that Im approaching this question correctly, I feel as though ive hit a bit of a wall with it. Perhaps I am trying to use the wrong formula?
    Last edited: Jan 14, 2015
  5. Jan 14, 2015 #4


    Two equations, two unknowns.

  6. Jan 14, 2015 #5
    Incidentally, for a constant temperature process, what is the equation for the change in entropy as a function of the volume ratio?

  7. Jan 14, 2015 #6
    Oh I see, so you then end up with Cv= R/ γ-1. Very helpful, thanks!
    Do you mean S2-S1 = Cv loge ((P2V2γ) / (P1V1γ))?
  8. Jan 14, 2015 #7
    No, I mean ##ΔS=nR\ln(V_2/V_1)##

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted