Calculating final pressure

1. Oct 26, 2011

fiziks09

1. The problem statement, all variables and given/known data

As a 1.00 mol sample of a monoatomic gas expands adiabatically the work done on it is
-2.50x10^3J. The initial temperature and pressure of the gas are 500k and 3.60atm. Calculate
a. The final temperature and b. The final pressure.

2. Relevant equations

First law of thermodynamics
Eint = Q + W

3. The attempt at a solution
I was able to solve the first part of the question. Since it was an adiabatic process Q = 0 and then used Eint = nCv(Tb - Ta) to get the final temperature which is 300k.

But i can't seem to get the final pressure. I have no idea on how to begin. Please any help will be very much appreciated.

2. Oct 26, 2011

Thaakisfox

Use the state equation for an adiabatic process i.e. p*T^(k/(1-k))=constant. Where k is the quotient of the heat capacities.

3. Oct 26, 2011

fiziks09

quotient of heat capacities?? please i don't understand that part!!!!

4. Oct 26, 2011

Thaakisfox

Its just that k=C_p/C_v. The quotient of the heat capacities at constant pressure and constant volume.

5. Oct 26, 2011

fiziks09

okay..i get that part.. is the constant R??

6. Oct 26, 2011

Thaakisfox

No... that equation means that this quantity p*T^(k/(1-k)) is constant throughout the process. Which means that its the same in the beginning and at the end. U know the pressure and temperature at the beginning you know tht temp at the end...

7. Oct 26, 2011

fiziks09

ok.. but i'm getting a wrong answer which is 5.9atm but it should be 1atm...

8. Oct 26, 2011

Thaakisfox

Are you making your calculations correctly? whats the value you got for k for the gas?

9. Oct 26, 2011

fiziks09

i got 1.5
i used C_v to be 3/2R since the gas is monoatomic and C_p to be 5/2R

10. Oct 26, 2011

Thaakisfox

shouldnt it be 1.667? And then whats the next step? show the whole calculation otherwise i cant see wtz wrong...