2012 practice final#2A

[Lily Wei 1D]

For this problem, why is the change in entropy calculated using only
and not ? The problem states that "The He in the metal case is initially held at a pressure of 1.0 atm", so doesn't that mean that the pressure is going to change and we should take that into account?

[Irena Roy 4H]

The question gives you no other information that could lead you to conclude that the pressure did actually change in the process of He leaking.

[Alex Nguyen 3I]

Using the equation with volume change already accounts for the pressure change because of PV=nRT. Therefore you can use one or the other when finding the entropy. Only when there's a temperature change with a volume/pressure change do you need 2 equations with a combined entropy.

[Suraj Palaparty 2F]

The case is initially at 1.00 atm, taking into account the change in pressure later in the problem, while the safe is isolated.