A tank of 50 ft^3 volume contains saturated ammonia at a pressure of 200 lbf/in^2. Initially the tank contains 50% liquid and 50% vapor by volume. Vapor is withdraw from the top of the tank until the pressure is 100 lbf/in^2. Assuming that only vapor (i.e., no liquid) leaves and that the process is adiabatic, calculate the mass of ammonia withdrawn.