## Homework 8.27

704564167
Posts: 20
Joined: Fri Sep 25, 2015 3:00 am

### Homework 8.27

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

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
Posts: 23
Joined: Wed Sep 21, 2016 2:58 pm

### Re: Homework 8.27

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
Posts: 60
Joined: Fri Sep 28, 2018 12:16 am

### Re: Homework 8.27

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

### Re: Homework 8.27

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

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.