Ideal gases, Cp and Cv

[Jorja De Jesus 2C]

What is the purpose of the fractions multiplied by R? For example Cp= (5/2)R; does this get plugged into or does it get put into U=3/2 nRT except the 3/2 would be 5/2? Also when do you know if it is Cp or Cv?

[Rachel Yu 1G]

Cp and Cv are the heat capacities for monatomic ideal gases at constant pressure and constant volume respectively. It would get plugged in when you need heat capacity (i.e. q=nCpdeltaT or q=nCvdeltaT). On the other hand, Cp or Cv does not replace R in PV=nRT.

[Juliana Chopelas 1A]

The fractions just represent the molar heat capacity for constant pressure and constant volume when plugging it into an equation. Also I think the question would specify "at constant volume" or "at constant pressure" to know when to use it.