HW3 common mistakes

Here were some of the common issues on HW 3:

1. Some had trouble integrating dW = PdV to find the work done under various conditions. Be sure to know when pressure is constant and when it varies (and how). On Problem 11, many people failed to realize that the system is adiabatic and hence there is no change in temperature/internal energy since it is an ideal gas. On Problem 12, many people tried to find W as a function of P1 and P2, i.e. they didn't know which variable to substitute between P and V (several people also tried to substitute V as f(P) after integration).

2. There were many 'intuitive' answers for Problem 9 based on Raoult's law etc. Credit was given if the reasoning was accurate, but the phase rule is a more general and powerful explanation since it can be applied to ternary and higher systems etc.

3. On Problem 13, there was some confusion about the sign for W.