## 8.27-V needed to calculate n?

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

sofiakavanaugh
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Joined: Thu Jul 13, 2017 3:00 am
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### 8.27-V needed to calculate n?

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, you need to use PV=nRT to solve for n to use in the work equation. My question is, how do you know which volume to input? In the solutions it uses the initial volume and therefore the initial moles, but how does that make sense conceptually if what you are calculating creates a change in moles?

Thanks

Luis De La Cruz 1H
Posts: 37
Joined: Fri Sep 25, 2015 3:00 am

### Re: 8.27-V needed to calculate n?

Like the part b states, it is an isothermal process, so by looking at the equation PV=nRT, you see how R and T remain constant so the only thing that changes is the number of moles which you said you are calculating so, by definition the volume has to change,again seen by P(delta V)= (delta n)RT, the change in moles is creating a change in volume, either increasing or decreasing depending on how the number of moles is changing.