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Question:

The specific heats, CP and CV, of a gas of diatomic molecules, A, are given (in units of Jmol⁻¹K⁻¹) by 29 and 22, respectively. Another gas of diatomic molecules, B, has the corresponding values 30 and 21. If they are treated as ideal gases, then?

A has one vibrational mode and B has two

Both A and B have a vibrational mode each

A is rigid but B has a vibrational mode

A has a vibrational mode but B has none

Solution:

Correct option is D. A has a vibrational mode but B has none
For A:
R = Cp - Cv = 7
Cv = fR/2 = 22
=> f = 44/7 = 6.3
For B:
R = Cp - Cv = 9
Cv = fR/2 = 21
=> f = 42/9 = 4.67