# The final temperature of a gas?

1. Oct 15, 2011

### JustinLiang

1. The problem statement, all variables and given/known data
Starting with 2.50 mol of N2 gas (assumed to be ideal) in a cylinder at 1.00 atm and 20C, a chemist first heats the gas at constant volume, adding 1.52x10^4J of heat, then continues heating and allows the gas to expand at constant pressure to twice its original volume. Calculate the final temperature of the gas.

2. Relevant equations
PV=nRT
ΔU=Q-W
ΔU=nCvΔT

3. The attempt at a solution
The first part suggests that the system is in a constant volume. Thus:
ΔU=Q
nCvΔT=Q
(2.5mol)(5/2)(R)(Tf-20)=1.52x10^4

I solve for Tf and I get 312.5C.

Now the next part suggests the system is at a constant pressure and the volume doubles.
So I use PV=nRT where n, R and P are constant.
V/T=V/T
1/312.5C=2/T

I solve for and I get 625C. However the answer in the back is 899C. What am I doing wrong?

Thanks!

2. Oct 16, 2011

### Redbelly98

Staff Emeritus
For this equation, T must be expressed as an absolute temperature, not in degrees C.