Hydrogen (viscosity = 0.009 centipoise) is pump from a reservoir at 2x10³ kPa pressure through a horizontal commercial steel pipe (50-mm diameter) and 500 meters long. The pressure of this gas is raised to 2.6 x 10³ kPa by a pump at the upstream end of the pipe and the downstream pressure is 2.0 x 10³ kPa. The conditions of flow are isothermal and the temperature of the gas is 293K. The mass velocity in kilograms per meter² per second is _____ kg/m²-s.