This is the example 2.12 from "Introduction to Chemical Engineering Thermodynamics", 6th. ed, by Smith, Van Ness & Abbott: "An evacuated tank is filled with gas from a constant-pressure line. What is the relation between the enthalpy of the gas in the entrance line and the internal energy of the gas in the tank? Neglect heat transfer between the gas and the tank." The solution is that the internal energy of the gas in the tank is equal to the enthalpy of the gas in the entrance line. Although I could solve the exercise, I can't say I've really understood/accepted the result. Two objections I have to it: a) the gas filling the evacuated tank is not doing any work, so it seems to me that the product PV (H = U + PV) is of no interest here. b) I can imagine that the entrance line is connected to a huge tank that supplies gas at the same pressure available at the line and that the filling of the evacuated tank will have a neglegible effect over that huge tank. When the flow ceases, the gas at both reservoir will have the same properties, equal to the original ones of the huge tank. As a consequence, the internal energy of the gas in the tank will be equal to the internal energy of the gas in the entrance line, and not equal to its enthalpy. Can anyone please clarify which concept I have not understood correctly?