Cp vs Cv

[Emily Tam 1k]

How do you know when to use Cp vs Cv?

For example, I am doing a problem where the helium balloon increases from 3.0L to 3.5L and the helium behaves as an ideal gas. Why would you use Cv when solving for the problem instead of Cp when volume is not constant?

[Marina Gollas 1A]

When I went to one of the review sessions, one of the UAs had stated that we use whichever one we are focusing on in the current problem and whichever one you use in your change in entropy equation, since you take into consideration each entropy calculation separately, since there are 2 delta S that we need to find in the problem.