For a rigid diatomic molecule, universal gas constant R = n C_p...

For a rigid diatomic molecule, the number of degrees of freedom is $f = 5$.

The molar specific heat at constant volume is $C_v = \dfrac{f}{2} R = \dfrac{5}{2} R$.

The molar specific heat at constant pressure is $C_p = C_v + R = \dfrac{5}{2} R + R = \dfrac{7}{2} R$.

Given the relation $R = n C_p$.

Substituting the value of $C_p$, we get R = n \left( \dfrac{7}{2} R \right)$.<br><br>Solving for$n$, we get$n = $\dfrac{2}{7}$.$<br><br>Answer: 2/7$