Air passes from a large reservoir at $70^{\circ} \mathrm{F}$ through an isentropic converging-diverging nozzle into the atmosphere. The area of the nozzle throat is $1 \mathrm{~cm}^{2}$, and that of the exit is $2 \mathrm{~cm}^{2}$. What is the reservoir pressure at which the flow in the nozzle just reaches sonic velocity, and what are the mass flow rate and exit Mach number under these conditions?