A cylinder with a moveable piston is completely filled with a small amount (100 millimoles) of liquid water at a pressure of 1.0
atm and a temperature of 80∘C. All the air in the cylinder is excluded. The cylinder is placed in a water bath held at 80∘C. The piston is slowly moved out to expand the volume of the cylinder to 20L as the pressure inside the cylinder is monitored. A plot of the pressure versus volume for the system is shown in the figure above. Which of the following statements most closely indicates, with justification, the region of the curve where the equilibrium represented below occurs?
Region B, because the pressure inside the cylinder is equal to the vapor pressure of water at 80∘C when both liquid and gas phases are present.
Explanation:
As expansion occurs, liquid water evaporates reversibly, holding the pressure constant at the equilibrium vapor pressure of water at 80∘C(0.47atm) 80∘C (0.47 atm). When all of the liquid has evaporated, the pressure drops and follows the ideal gas law.
