A closed cylinder and piston system contains only liquid CS2 and its vapor in equilibrium at 28 C. The volume the vapor occupies is 300 mL and the pressure of the vapor is 400 torr. The piston is then moved so that the vapor occupies 600 mL, and the system is allowed to come to equilibrium at 28 C. What is the final pressure of the CS2 in the vapor phase?