Reversible adiabatic expansion I am confused with adiabatic expansions.  I have a homework problem wherein 2.75 moles of an ideal gas at 375 K expands adiabatically with an initial pressure of 4.75 bar and a final pressure of 1.00 bar with a C(p,m)=5R/2. I need to calculate the work done by the expansion. 
Where do I start (I don't want just the answer, as I could have just copied that from the back of the book)?
 A: Adiabatic means that there was no heat exchange with the environment $\Delta Q=0$. Thus, by the 1st law of thermodynamics, the work done by the gas is equal to the change in internal energy: $A=\Delta U$. Since this is an ideal gan, it's internal energy is function of temperature only. You just need to use the right formula for the internal energy...
EDIT (inspired by comments): This solution uses only the energy conservation condition and does not require the process to be reversible. In a reversible process, there is no internal entropy production and, if there is also no heat exchange, the entropy of the gas remains constant. Thus an adiabatic reversible process is necessarily isoentropic.  Sometimes the word "adiabatic" is understood as a synonym to conservation of entropy. However, reversible or not, the external work must be done at the expense of the internal energy if no heat is exchanged. Hence in your  problem "adiabatic" must mean "no heat exchange", otherwise no unique solution is possible.
