In thermodynamics
Change in Internal Energy = Heat - Work (\DeltaU=Q-W)
During vaporization my Heat is mass*Latent Heat (Q=mL) and Work is Pressure*change in volume (W=P\DeltaV), so \DeltaU=mL-P\DeltaV.
Let's take any of the natural monoatomic gases, Neon in this case.
\Delta = Volume...