1. Dec 8, 2003

### tristan_fc

I've been struggling with this problem for an hour or so now, and can't seem to find the right answer. Perhaps someone here can help? I would be very grateful. ;)

Two moles of an ideal gas ( = 1.40) expands slowly and adiabatically from a pressure of 5.09 atm and a volume of 12.7 L to a final volume of 30.0 L.

I had to find this stuff first, and I know I have all of it right.

final pressure: 1.528
initial temp: 393.5K
final temp: 279

Find Q, W, dEint.

I know Q is 0, and I know that Work and change in internal Energy are the opposite of eachother, but I can't seem to find the right value for them. I thought work = nRdT, which in this case is:

2*.08214*(393.5-279)=1902.9

However, that answer is wrong according to webassign. There is also a problem in my text book that is the same, except with different numbers, and i tried that one and got it wrong too. Anyone know what I'm doing wrong? Your help is greatly appreciated. :)

2. Dec 8, 2003

### himanshu121

Formula for work done in adiabatic process is wrong

3. Dec 8, 2003

### tristan_fc

so what is the proper formula? The textbook goes through a proof of why PV^y is constanst, but doesn't really go any further from that... I'm guessing that's somehow incorperated into the formula...

4. Dec 8, 2003

### himanshu121

Write the differential equation for for work done for gas
i.e
dW=PdV

Note $$PV^\gamma= K$$

substitute P from above in workdone equation and calculate the work done by taking limits from V1 to v2

5. Dec 8, 2003

### tristan_fc

eh... P from above what? P is not constant.

6. Dec 8, 2003

### himanshu121

Refining it
$$dW=kV^-^\gamma dV$$

I hope u will got it now

Last edited: Dec 8, 2003
7. Dec 8, 2003

### tristan_fc

Okay, I thought maybe that's what you meant, but I'm sorry I don't see what good that does if I don't know what k is. Maybe I'm just slow tonight as I only got about 4 hours of sleep last night and am rather tired... Sorry. [zz)]

Ah well, it was due at midnight. So I'll just ask my teacher how to do it tomorrow...

Last edited: Dec 8, 2003
8. Dec 8, 2003

### himanshu121

k is a constant which is given by

$$PV^\gamma= k$$

$$P_iV_i^\gamma = P_fV_f^\gamma= k$$

Last edited: Dec 8, 2003
9. Dec 10, 2003

### tristan_fc

Oh duh. Sorry, I guess I was just really tired last night. ;) Anyway thanks for the help.