The final temperature of a gas?

[JustinLiang]

Homework Statement

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.

Homework Equations

PV=nRT
ΔU=Q-W
ΔU=nCvΔT

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!

 

[Redbelly98]

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

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