Determining concentration by measuring pressure change

[Boltzano]

Homework Statement

Derive an equation how one can calculate concentration by measuring pressure change in a known volume.

Known data:
Pressure change dp/dt in a known volume V, volumetric flow rate F, total pressure p and temperature T.

Homework Equations

Probably ideal gas law pV=NkT and mass volume rate (throughput) Q=Fp=d(pV)/dt

The Attempt at a Solution

Not really sure how to solve this since this was not an textbook question but actual measurment so I'm not sure about the needed equations. But I would start:

d(pV)/dt=V*dp/dt=Q=Fp (since V is constant)

=> V*dp/dt=(Nkt/p)*(dp/dt)=Fp

=>N=(Fp^2/kT)*(dt/dp)

But doing so I end up with number of molecules and not concentration. This could easily be solved by dividing with known volume V but for some reason I won't get right numerical concentration (in measurments computer program calculated the concentration automatically from known data).

 

[Boltzano]

Was there some information missing or is this just too simple so people won't bother =D

Somehow my solution just doesn't seem right. Is it ok to take out the V from d(pV)/dt and then substitute with NkT/p?