I had not learned about this topic in my few years of study, so it is quite possible that I am incorrect with my input which follows. Excuse me if that is the case. Having used the relation which tamim83 offered, it seems that the solution which you listed, that is, the Hertz-Knudsen equation- which, for some reason, no longer appears when I view your original post- is not displayed correctly. My search for the Hertz-Knudsen equation online confirmed that what you previously posted as the equation was only slightly incorrect. It should read:

$$ n = P \sqrt {\frac {N_{A}} {2 \pi m R T}}/$$

Since it no longer appears in your post, I cannot be certain that I read it correctly (that is, being written incorrectly in your post). If I was mistaken, please accept my apology.

-Matt