Q. An ideal gas is made of polyatomic molecules. Each of the molecules has three translational, three rotational and f number of vibrational modes. If the ratio of heat capacities $C_P/C_V$ of the gas is 8/7, then the value of f is:
A) 1
B) 4
C) 3
D) 2
Explanation
[Solution from source] Each vibrational mode contributes R to $C_V$: $C_V=(3+f)R$, $C_P=(4+f)R$. Setting $(4+f)/(3+f)=8/7$ gives 28 + 7f = 24 + 8f, so f = 4. Answer (2).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.