First Law - Open Systems: Example Problems

Try to solve these problems before watching the solutions in the screencasts.

Example Problem 1

One kg of liquid water at 40°C is in a flask that is connected, through a valve, to a vacuum pump. Water evaporates when the valve is opened. The water vapor is removed fast enough that the flask can be considered adiabatic. As water evaporates, the remaining liquid water cools, and when it reaches 0°C, it starts to freeze. What fraction of the water will have evaporated when the rest is frozen?



Example Problem 2

An empty tank is filled by opening a valve and allowing steam at 450°C and 10 MPa to flow into it until the pressure equalizes. What is the temperature in the tank when the pressure equalizes?