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Caitlin Mispagel 1D
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Joined: Tue Oct 10, 2017 7:13 am


Postby Caitlin Mispagel 1D » Tue Jan 30, 2018 8:23 pm

For this question it asks "Calculate the change in entropy when the pressure of 1.50 mol Ne(g) is decreased isothermally from 15.0 atm to 0.500 atm. Assume ideal behavior." I am using the equation deltaS= nRln(P2/P1) but I am getting the wrong answer. I found that the problem was that to get the right answer I need to use 15.0 as P2 and 0.500 as P1 but since 0.500 is the final and 15.0 is the initial why wouldn't it be the other way around?

Sabrina Dunbar 1I
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Joined: Fri Sep 29, 2017 7:07 am
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Re: 9.11

Postby Sabrina Dunbar 1I » Tue Jan 30, 2018 8:36 pm

I got the correct answer taking the natural log of (.500 atm/15 atm) and then multiplying it by the constant value R and 1.5 as the number of moles of neon. I will include the substituted values that I used below. Maybe it was a simple calculation error?

∆S=(1.5 mol)(8.314 J/Kmol)(ln(.500 atm/15 atm))=42.4 J/K

Lena Nguyen 2H
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Joined: Fri Sep 29, 2017 7:06 am
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Re: 9.11

Postby Lena Nguyen 2H » Tue Jan 30, 2018 8:38 pm

You're correct about the initial and final pressures, but the equation is deltaS = nRln(P1/P2) according to the solutions manual.

Mitch Mologne 1A
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Joined: Fri Sep 29, 2017 7:04 am

Re: 9.11

Postby Mitch Mologne 1A » Tue Jan 30, 2018 10:10 pm

I believe it is because the starting equation is deltaS=nRTln(V2/V1) with volumes instead of pressure. If we apply Boyle's law that Pressure and Volume are inversely proportional (V1P1=V2P2) then we would be able to inverse the volumes to be pressures, thus deltaS=nRTln(P1/P2). To my knowledge, that is why you were able to get the right answer by having the pressures flipped.

Morgan Baxter 1E
Posts: 50
Joined: Thu Jul 27, 2017 3:00 am

Re: 9.11

Postby Morgan Baxter 1E » Wed Jan 31, 2018 12:31 am

You are correct in thinking that the initial pressure (P1) is 15 and the final pressure (P2) is .5. However, you are using an incorrect equation, which resulted in the wrong answer. The equation is change in entropy= nRln(P1/P2). The equations for change in volume and temperature have ln(V2/V1) and ln(T2/T1) respectively, however the pressure equation has ln(P1/P2) because according to Boyle's law, pressure is inversely proportional to volume. Therefore, if the volume equation is: change in entropy= nRln(V2/V1), then substituting pressure for volume according to boyles law, you get change in entropy= nRln(P1/P2). Once you use the correct formula, you will get the correct answer.

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