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Homework 8.27

Posted: Sun Jan 10, 2016 6:36 pm
by 704564167
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?

Re: Homework 8.27

Posted: Sun Jan 10, 2016 8:18 pm
by Sandeep Gurram 2E
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.

Re: Homework 8.27

Posted: Wed Mar 15, 2017 1:55 am
by Hector_Gutierrez 1J
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?

Re: Homework 8.27

Posted: Wed Jan 30, 2019 5:46 pm
by klarratt2
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.

Re: Homework 8.27

Posted: Tue Feb 11, 2020 3:47 pm
by 005321227
I'm a bit confused as to where the given temperature comes in

Re: Homework 8.27

Posted: Tue Feb 11, 2020 4:00 pm
by 005321227
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.