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Chemistry Forums for Students => Physical Chemistry Forum => Topic started by: dfodor on July 04, 2010, 04:11:22 AM

Title: Hertz-Knudsen equation
Post by: dfodor on July 04, 2010, 04:11:22 AM

I would have a question to Hertz-Knudsen equation. The following equation comes from the integration of velocity distribution:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fimg88.imageshack.us%2Fimg88%2F575%2Fvelo.gif&hash=f260a0f23fbd3ce680dce433e1c83e77acfb0bd6)
where D is density of the gas. Using the following relations:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fimg688.imageshack.us%2Fimg688%2F4839%2Feq1.gif&hash=9266a391a7974ee5605d94f098390149714fe13b), (https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fimg204.imageshack.us%2Fimg204%2F1856%2Feq2.gif&hash=d43dbcf653ddd7ebf1d5ec0490fd43c6c49e16e4) and (https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fimg822.imageshack.us%2Fimg822%2F8374%2Feq3.gif&hash=39026db093fbdf9efb7169aab77d67bd685ac0a1) we come at the end to the Hertz-Knudsen equation:
(https://www.chemicalforums.com/proxy.php?request=http%3A%2F%2Fimg408.imageshack.us%2Fimg408%2F8453%2Feq4.gif&hash=72341a29840913bccb1ebaadf9640885a818727b).
I do not see how we come to the last formula. Could you help, please?

Title: Re: Hertz-Knudsen equation
Post by: tamim83 on July 15, 2010, 08:23:26 AM

R = N_Ak , that should help you some.  Remember the rule for dividing powers as well. Also, do you mean to use a lowercase "m"?  "M" usually means "molar mass", which could cause some confusion.  

Title: Re: Hertz-Knudsen equation
Post by: haz658 on July 18, 2010, 08:01:42 PM

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

Title: Re: Hertz-Knudsen equation
Post by: dfodor on July 19, 2010, 11:03:46 AM

Thank you for the replies. I copied it correctly. Uppercase and lowercase "m"-s are correct. I hate posting such things but in this case there must be something wrong with the manuscript.