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 Homework Statement:
 We have 5.32 L of an ideal diatomic gas at 16.3 bar and 371 K. The gas is in an insulated cylinder contained with an insulated piston. We unlock the piston and the gas expands against a constant external pressure of 1.43 bar until the piston is locked again at triple the original volume. Calculate the values of the parameters below for this process. Express all energies in J and entropy in J/K.
 Relevant Equations:

W= PexdV
delta U = q+ w
ds= dq/T
I have been able to get everything except entropy. I know it's not zero. I know I have to find a reversible path to calculate it, but keep coming up with strange values so I don't think i'm doing it correctly.
can I do CpdT/T + CvdT/T = ds? Im having trouble calculating my P2 (I know my final pressure is not the constant external pressure) and T2.