[G O D I V A]

Below is the whole question.

What I'm asking is the second part of part b - calculating the enthalpy change in the tank.

I've done and and the first part of b so basically I know that:

Initial temperature is 300K, final temperature is 226K
Initial pressure is 1 atm, final pressure is 0.1 atm
Enthalpy change of molecules that left the tank is 0.

How do I calculate the change in enthalpy of the molecules still in the tank?