## Q. 8.27

$\Delta U=q+w$

Lauren Huang 1H
Posts: 60
Joined: Fri Sep 28, 2018 12:16 am

### Q. 8.27

Question 8.27 from the 6th edition says:

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 a, why does the internal pressure (1.79atm) not matter?
And for part b, why is it that the volume of 4.29L was used to find moles of the gas instead of the final volume 6.52L?

Ashley Kim
Posts: 62
Joined: Fri Sep 28, 2018 12:19 am
Been upvoted: 1 time

### Re: Q. 8.27

The internal pressure does not matter because the only pressure that is inputted into the equation is the pressure that contributes to the work to the system - i.e. the external pressure.
The initial volume was used to find the moles of gas because it is the initial state that can be accurately plugged in with the temperature value and pressure (assuming you used the equation PV=nRT).