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Homework Help: Calculating Entropy Change in an expansion

  1. Oct 26, 2014 #1
    1. Suppose that you have a sample of a gas in a cylinder equipped with a piston that has a volume of 1.50 L, a pressure of 1.20 atm, and a temperature of 250 K. Suppose that the gas is expanded reversibility under isothermal conditions until the pressure is 0.75 atm. What is the entropy change that accompanies this expansion?

    2. Relevant equations
    dS= nRln (V2/V1)

    3. The attempt at a solution
    So I calculated V2 using p1V1=p2V2 and got 2.4L. I plugged that into the equation above but got 3.9...The answer is suppossed to be 0.34. Can you guys help me? :)
  2. jcsd
  3. Oct 26, 2014 #2
    What are the units of your answer, and what are the units of the accepted answer? What value did you get for n?
  4. Oct 26, 2014 #3
    The units are 0.34 J/K and as for the moles I think we were to assume the gas to be one mole.
  5. Oct 26, 2014 #4
    Oh Okay I got it.... so I was to calculate n too...duh...Thanks!!! Got it.... *Feeling enlightened*
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