## Which P to use? 6th edition, 8.27

$w=-P\Delta V$
and
$w=-\int_{V_{1}}^{V_{2}}PdV=-nRTln\frac{V_{2}}{V_{1}}$

Tyra Nguyen 4H
Posts: 74
Joined: Fri Sep 28, 2018 12:25 am

### Which P to use? 6th edition, 8.27

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

For part b, how do I know which P to use? Still confused about that for irreversible vs reversible expansions.

Brandon_Tran_2E
Posts: 63
Joined: Fri Sep 28, 2018 12:23 am

### Re: Which P to use? 6th edition, 8.27

Irreversible expansion uses the equation w = -PexΔV where you would plug in for pressure.

For reversible expansion you would use w = -nRT x ln(vf/vi)... one of the TA's said that questions will almost always tell you what type of expansion it is, therefore you would use whatever equation is linked to that situation

Rian Montagh 2K
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Joined: Fri Sep 28, 2018 12:15 am

### Re: Which P to use? 6th edition, 8.27

For b) the equation is w= -nRTln(Vf/Vi), and to find n you use PV=nRT and use the original given P value I believe (1.79 atm).

Alexa Tabakian 1A
Posts: 38
Joined: Fri Sep 28, 2018 12:20 am

### Re: Which P to use? 6th edition, 8.27

Yes, the pressure you would use to find n is 1.79 atm.