physics
posted by Annie .
We start with 5.00 moles of an ideal monatomic gas with an initial temperature of 123C. The gas expands and, in the process, absorbs an amount of heat equal to 1300J and does an amount of work equal to 2040J.
What is the final temperature of the gas?
Use R= 8.3145J/(mol*K) for the ideal gas constant
I'm not sure how to set this up. I tried a few ways, but all of them ended up with the wrong answer

The change in internal energy (U) is equal to heat in (Q) minus work out (W).
The key to this probloem is knowing that the internal energy per mole of a MONATOMIC gas is (3/2) R T.
Thus,
delta Q = (3/2)*n* R*(delta T)
= 1300  2040 = 740 J
n is the number of moles and R = 8.317 J/mole K
Solve for delta T. It will be negative. I get 12 C (or K) 
Hey thanks! At one point I was using (3/2)R(deltaT) but I wasn't setting it equal to (DeltaQ).