Answer
20 moles
Work Step by Step
Recall, a diatomic gas has 5 degrees of freedom, so its value of $C_v=2.5R$.
Thus, we find the difference in the two values of $C_v$.
$ \Delta C_v = 3R-2.5R = .5R$
Thus, it follows:
$n =\frac{10}{.5}=20 \ moles$
