A well-insulated 2-m³ rigid tank contains saturated ammonia at 40 ˚C. Initially, the tank contains 50% (volume based) of this ammonia in liquid form and the rest (50%) in vapor form. The valve (on the tank) is opened, discharging ammonia vapor (not liquid) until the temperature reaches 10 ̊C. How much is the mass of discharged ammonia vapor?