Carbon dioxide sublimes at pressures below roughly 5 atm?

At a pressure of 2 atm this phase transition occurs at about ?69 ^?C with an enthalpy of sublimation of roughly 26 kJ/mol. Suppose a kilogram of solid CO₂ at a temperature of ?69^??C is confined in a cylinder by a piston that exerts a constant pressure of 2 atm. How much heat must be added to completely convert it to gas? Assuming CO₂ to be an ideal gas, how much work was done by the CO₂ in the course of vaporizing? If the ambient pressure outside the cylinder is 1 atm, how much useful work was done? Finally, by how much did the internal energy of the CO₂ change when it vaporized?

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109 10 107 solid liquid СР 106 ТР 105 vapor 104 180 200 220 240 260 280 300 320 temperature [K] pressure (Pa]