## HW problem 4B. 13

Gisela F Ramirez 2H
Posts: 61
Joined: Fri Sep 28, 2018 12:27 am

### HW problem 4B. 13

Calculate work for following process beginning with gas sample in a piston assembly with T=305K, P=1.79 atm, and V=4.29 L
a.) Irreversible expansion against a constant external pressure of 1 atm to a final volume of 6.52 L
I was able to calculate process a, -226 J but for

b.) Isothermal, reversible expansion to a final volume of 6.52L

I tried using the equation w= -nRT ln(V2/V1) but I did not end up with the correct number. Would I assume that I am dealing with one mole?

ThomasLai1D
Posts: 61
Joined: Fri Sep 28, 2018 12:17 am

### Re: HW problem 4B. 13

Because PV=nRT, you can calculate the amount of moles by plugging in the given information into the equation n=PV/RT. Using the value you found for n, you should be able to use the w=-nRTln(V1/V2) formula.

abbydouglas1K
Posts: 65
Joined: Fri Sep 28, 2018 12:26 am

### Re: HW problem 4B. 13

How do we know when we to use ln(V1/V2) ?

Sara Lakamsani 4D
Posts: 61
Joined: Fri Sep 28, 2018 12:29 am

### Re: HW problem 4B. 13

We use the second equation when the external pressure is not constant, so in any reversible process

LG2019
Posts: 60
Joined: Fri Sep 28, 2018 12:27 am

### Re: HW problem 4B. 13

I am plugging the values into the w = -nRTln(V2/V1) equation but am not getting the right answer. Are my values correct?
n = (1.79atm)(6.52L)/(8.3145)(305K) = 0.00460 mol
w = -(.00460)(8.3145)(305)ln(6.52/4.29) = -488J
The answer in the book is -326J

Sara Lakamsani 4D
Posts: 61
Joined: Fri Sep 28, 2018 12:29 am

### Re: HW problem 4B. 13

Hope this helped!