Graph of specific heat at constant volume for a monoatomic gas is

According to first law of thermodynamics $\Delta$Q=$\Delta$U+P $\Delta$V If $\Delta Q$ is absorbed at constant volume, $\Delta V=0$ C_V=\left(\frac{\Delta Q}{\Delta T}\right)_V=\left(\frac{\Delta U}{\Delta T}\right)_V=$\frac{\Delta U}{\Delta T}$for an ideal monoatomic gas $\frac{\Delta U}{\Delta T}=\frac{3}{2}$R ; C_V=$\frac{3}{2}$R