if a gas expands isentropically (consider the process reversible adiabatic), from point 1 in a nozzle to point 2 downstream of the nozzle. If you know the pressure and temperature at point 1, can you use the relationship pv=1/M RT to work out the specific volume at point 1? And similarly if you know pressure at point 2 and temp at point 2, can you use Pv= 1/M RT to work v2? Or do you have to use the equation Pv^lamba = constant. The thing is you don't know the constant here.