Homework 8.27

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704564167
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Joined: Fri Sep 25, 2015 3:00 am

Homework 8.27

Postby 704564167 » Sun Jan 10, 2016 6:36 pm

Calculate the work for each of the following processes beginning with a gas sample in a piston assembly with T=305K, P=1.79atm, and V=4.29L:
(a) irreversible expansion against a constant external pressure of 1.00atm to a final volume of 6.52L
(b) isothermal, reversible expansion to a final volume of 6.52L.

I understand that for (a) you must use w=-P(change in volume) and for (b) you use w=-nRT(V2/V1) but how do you know when to use each equation?

Sandeep Gurram 2E
Posts: 42
Joined: Fri Sep 25, 2015 3:00 am

Re: Homework 8.27

Postby Sandeep Gurram 2E » Sun Jan 10, 2016 8:18 pm

For problems involving irreversible expansion you would use w = -PexΔV.

For problems involving reversible, isothermal expansion, you would use w = -nRT x ln(vf/vi)

The problem usually specifies the type of expansion you're dealing with.

Hector_Gutierrez 1J
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Joined: Wed Sep 21, 2016 2:58 pm

Re: Homework 8.27

Postby Hector_Gutierrez 1J » Wed Mar 15, 2017 1:55 am

For part b, I got -1061.4 J. I used the equation -nRTln(Vf/Vi). I set n=1, 8.314 as the gas constant R, 305 K as my temperature T, my final volume as 6.52 L, and my initial volume as 4.29 L. However, the answer at the back of the book is -326 J. What did I do wrong?

klarratt2
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Joined: Fri Sep 28, 2018 12:16 am

Re: Homework 8.27

Postby klarratt2 » Wed Jan 30, 2019 5:46 pm

Hector_Gutierrez 1J wrote:For part b, I got -1061.4 J. I used the equation -nRTln(Vf/Vi). I set n=1, 8.314 as the gas constant R, 305 K as my temperature T, my final volume as 6.52 L, and my initial volume as 4.29 L. However, the answer at the back of the book is -326 J. What did I do wrong?


n does not equal 1. You have to use the given conditions to calculate n with the equation PV = nRT.

005321227
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Joined: Sat Sep 07, 2019 12:15 am

Re: Homework 8.27

Postby 005321227 » Tue Feb 11, 2020 3:47 pm

I'm a bit confused as to where the given temperature comes in

005321227
Posts: 90
Joined: Sat Sep 07, 2019 12:15 am

Re: Homework 8.27

Postby 005321227 » Tue Feb 11, 2020 4:00 pm

Using pv=nrt, I determined that n=0.003mol. However, when I put that into the equation w=-nRTln(v2/v1), I get -3.18J, which is not possible since an isothermal reversible expansion always does more work than an irreversible one. Can anyone determine what I did wrong? I'm having trouble figuring it out.


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