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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 :

Asked in Re-NEET 2026 · Vibrational modes

Answer: (1) 4

Step-by-step solution

Each translational and rotational mode contributes 1/2R to C_V; each vibrational mode contributes R (kinetic and potential terms).

C_V=3/2R+3/2R+fR=(3+f)R, and C_P=C_V+R=(4+f)R.

(4+f)/(3+f)=8/7: 28+7f=24+8f, so f=4.

Why the other options are wrong

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