DeltaS= nCln(T2/T1)

[Shirley Liu 2I]

DeltaS= nCln(T2/T1) is used to calculate entropy change when the temperature changes. However how would we know when to use Cv or Cp for C??

[whitney_2C]

You know to use C_v or C_p depending on what the givens are in the question. If the system is at constant pressure then use C_p and if it is at constant volume then you use C_v.

[Matthew Mar 1J]

There are also some tricky ways that constant pressure or volume can be hinted at in the problem. For example, the question might not say constant pressure, but give you a value for enthalpy which in itself implies constant pressure. There are also problems where you have to split the entropy change into two steps and assume that either pressure or volume stay the same while you're making the temperature change calculation.

[anthony_trieu2L]

Always look at the context of the question. If you are having trouble determining which one is constant, go back to the basics. Like stated above, sometimes you may have to split the entropy change into several steps, which works since entropy is a state function.

[mbaker4E]

the question should always tell you if the system is at constant volume or constant pressure, in which you would use Cv and Cp, respectively.