Question 6

Consider a gas of triatomic molecules. The molecules are assumed to be triangular and made of massless rigid rods whose vertices are occupied by atoms. The internal energy of a mole of the gas at temperature T is:

Degrees of freedom for a non-linear (triangular) rigid triatomic molecule:

$$f = f_{\text{translational}} + f_{\text{rotational}} = 3 + 3 = 6$$

Internal energy of one mole of gas: $$U = \frac{f}{2}RT$$

$$U = \frac{6}{2}RT = 3RT$$

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