Dmixture of monoatomic and diatomic
d
$\mathrm{U}=\mathrm{U}_{0}+2 \mathrm{PV}=\mathrm{U}_{0}+2 \mathrm{nRT}$
$\mathrm{So}, \mathrm{dU}=2 \mathrm{nRdT}=\mathrm{nC}_{\mathrm{v}} \mathrm{dT}$
$\Rightarrow \mathrm{Cv}=2 \mathrm{R}\{\text { mixture of } \quad$ monoatomic
$\mathrm{Cv}=3 \mathrm{R} / 2 \& \text { diatomic } \mathrm{Cv}=5 \mathrm{R} / 2\}$