1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Unspecified gas, preasure known, looking for number density.

  1. Apr 27, 2010 #1
    1. The problem statement, all variables and given/known data

    Integrated circuits are manufactured in vacuum chambers in which the air pressure is 7.00×10^(−11) mm of Hg. What are (a) the number density and (b) the mean free path of a molecule? Assume T = 21.0 C.

    I've double checked, this is all the information available. Quoted verbatim.

    2. Relevant equations

    101,300 Pa = 760 mm of HG. I was able to figure the pressure in Pa. Pa = 9.33 x 10^(-9).

    I've seen various equations involving pressure and N (number density), but they all involve volume and other variables. p = (1/3)(N/V)(m)(v)^2 where v is the RMS velocity. I don't know the m value for "air", etc. I know N will be the number of molecules per cube meter.

    3. The attempt at a solution

    I figured the pressure in Pa, but I don't know which formula to use when they all seem to have several unknowns. Am I missing something, or is the question not specific enough?

  2. jcsd
  3. Apr 27, 2010 #2
    Ok. I see that pV = NkT. Where k is Boltzzmans (sp?) constant. T is temperature. Etc.

    I suppose I just assume that V is a cubic meter, since the units of number density are molecules per cubic meter?
  4. Apr 28, 2010 #3


    User Avatar
    Homework Helper

    Yes, you are on the right track. k is the Boltzmann constant, and you need the number of molecules in one cubic meter.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook