When to use Cv or Cp? to calculate change in Entropy

[Mirna_Velasco_1L]

I understand that we use Cv or Cp when the it states that its either at constant volume or pressure? But if these words are not used, and it just states that its a monatomic ideal gas, which C do use? I noticed that we normally use Cv but why?

[Paul Sedaros 1B]

When we use Cv, it is usually used when there is a change in temperature in the problem

[Maria Bajenov 1I]

It depends on the type of system the reaction is being carried out in. If the problem specifies that it is isothermal, use Cp. It it is being carried out in isochoric, use Cv. If it is isobaric, use Cp to calculate q because pressure is not changing and change in volume will be accounted for in calculating w.

[melissa carey 1f]

It depends on the type of system the reaction is being carried out in. If the problem specifies that it is isothermal, use Cp. It it is being carried out in isochoric, use Cv. If it is isobaric, use Cp to calculate q because pressure is not changing and change in volume will be accounted for in calculating w.

In the isobaric case, how would one find q?