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My copy of "Resnick and Halliday" states that

"$Cv$=$(f/2)R$ which agrees with the experimental evidence for diatomic gases($f$=5), but it is too low for polyatomic gases($f=6$).

Exactly what do they mean by the statement that I have highlighted in bold? What's too low? $Cv$? How come? Doesn't $Cv$ vary directly with $f$?

Please share your insights and help. MUCH thanks in advance :) Regards.

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It means that equation underestimates the experimental $C_v$ value for polyatomic gases. Indeed, the experimental $C_v$ value is about 15% larger than what is given by that equation (source).

This is because polyatomic gases have other vibrational degrees of freedom that must be included.

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