## Quiz 1 Prep Question 3

$\Delta U=q+w$

Ariana de Souza 4C
Posts: 99
Joined: Wed Sep 21, 2016 2:56 pm

### Quiz 1 Prep Question 3

IF 2 mol of an ideal gas at 300 K and 3 atm expands isothermally and reversibly from 6 L to 18 L and has a final pressure of 1.20 atm, what is w, q, and delta U?

What do we do if both volume and pressure are changing? What equations should I be considering? Thanks

Alyssa Chan 3B
Posts: 16
Joined: Wed Sep 21, 2016 2:57 pm

### Re: Quiz 1 Prep Question 3

Since both pressure and volume are changing (implying constant temp), you would use the equation W = -nRT$\ln\frac{Vf}{Vi}$. For all reversible reactions, ∆U = 0. You can use these two equations to solve for w, q, and ∆U.

Ariana de Souza 4C
Posts: 99
Joined: Wed Sep 21, 2016 2:56 pm

### Re: Quiz 1 Prep Question 3

right, because q=-w. But then using that equation, we don't factor in pressure. Do you ignore the values of pressure that are given in the problem?

Emma_Green_2C
Posts: 21
Joined: Wed Sep 21, 2016 2:57 pm

### Re: Quiz 1 Prep Question 3

I noticed Dr. Lavelle answered this question in another post. He just used the volumes and ignored the pressures given in the problem. The ratios of the pressures and the volumes should actually be the same, so you could use either V2/V1 or P1/P2 in the equation (since pressure and volume are inversely related). I think it's actually an error in the problem and the final pressure was supposed to be 1.0 atm not 1.2 atm because then the ratio for volume would be 18.00 L/6.00 L= 3 and and for pressure would be 3.00 atm/1.00 atm= 3.

Ariana de Souza 4C
Posts: 99
Joined: Wed Sep 21, 2016 2:56 pm

### Re: Quiz 1 Prep Question 3

okay, thanks Emma!