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**Summary::**Heat capacity for real gas with ideal gas (zero pressure) equation

I'm looking at this problem and I'm stuck.

I usually question everything but this problem is confusing me.

I don't know how they've made the jump from reduced properties (from generalized Cp charts(?)) to

**Cp-Cp°=1.70 cal/molK**

Usually Cp-Cv=R

I've searched online and it seems Cp-Cv=ZR

This is not the same as Cp-Cp°=ZR, which would mean they've made Cp°=Cv (zero pressure so this is true?)

Has the author got the compressibility factor from the generalised compressibility chart for the Pr and Tr (Z=0.92)

I'm confused how they've made something straight forward

Below is the problem plus the authors solution: