You are absolutely right, I should have used the formula for adiabatic expansion. However, I maintain that the pressure inside the thermos should only be 60atm

You are absolutely right, I should have used the formula for adiabatic expansion. However, I maintain that the pressure inside the thermos should only be 60atm, since this is the vapor pressure of CO2 at room temperature [1], where the liquid and the gas phase are in equilibrium. This is like the butane in a lighter: Butane evaporates at room temperature, but there is an equilibrium between liquid phase and gaseous phase when the pressure is higher. Using 60bar and the adiabatic formula, I get 12.5kJ of energy. [1]: http://en.wikipedia.org/wiki/Carbon_dioxide#Physical_properties http://en.wikipedia.org/wiki/Carbon_dioxide#Physical_propert...