Something that can not be used up or depleted

Oxygen gas enters well-insulated diffuser at 25 lbf/in.2, 420r, with a velocity of 860 ft/s through a flow area of 1.6 in.^2. at the exit, the flow area is 12 times the inlet area, and the velocity is 19 ft/s. the potential energy change from inlet to exit is negligible. assuming ideal gas behavior for the oxygen and steady-state operation of the diffuser, determine the following: (a) the exit temperature, in r(b) the exit pressure, in lbf/in.^2 (c) the mass flow rate, in lb/s