an air tank has a volume of 1.5 cubic meters with an initial pressure of 800 kpa and 20 c. it exhausts through a converging nozzle with a throat area of 0.75 square centimeters. what is the initial mass flow rate in kg/s? how long will it take for the pressure to reach 500 kpa? how long will it take before the nozzle flow ceases being choked?