1. The problem statement, all variables and given/known data A well-insulated rigid tank contains 7 kg of a saturated liquid-vapor mixture of water at 150 kPa. Initially, three-quarters of the mass is in the liquid phase. An electric resistance heater placed in the tank is now turned on and kept on until all the liquid in the tank is vaporized. Determine the entropy change of the system during this process. 2. Relevant equations 3. The attempt at a solution Using tables I found S1 = 2.88105 kJ/kg K by s=Sf + xSfg Since it finishes at the saturated vapor line the final entropy should be Sg @ 111.15 °C because this is the saturation temperature which I interpolated to 7.22369 kJ/kg K. For some reason the final value is completely wrong. The books gets it to 6.7296 for some weird reason. My thought was that if the box has a constant volume, the pressure is increasing so we can't use Tsat to find our final entropy. In other words - this process is not isothermal. Is this assumption correct?