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Homework Help: How many gas molecules

  1. Apr 28, 2010 #1
    An excellent laboratory vacuum has a pressure of 4.00x10^-18 atm, or 4.04x10^-13 Pa. How many gas molecules are there per cubic centimeter in such a vacuum at 297 K?

    n/v = p/rt

    n/v = 4.04x10^-13 / (8.31x297) = 1.66x10^-16 mol/m³

    1.66x10^-16 mol/m³ * (1/100cm)³ = 1.66x10^-22 mol/cm³

    but something is wrong, please verify and she what i am doing wrong
  2. jcsd
  3. Apr 28, 2010 #2


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    1 mole contains how many molecules?
  4. Apr 28, 2010 #3
    Last edited: Apr 28, 2010
  5. Apr 28, 2010 #4


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    And you have 1.66x10-22 moles, so how many molecules do you have?

    Actually now that I see it, you are finding per cm3.

    R has the units J/mol.K

    J = 1 Pa m3, so you'll need to change your units for R for it to work out properly.
  6. Apr 28, 2010 #5
    ok i see it now i forgot to multiply by (6.022x10^23/mol)

    i only got 1 more try at answering this question :(

    (1.66x10^-16 mol/m³) * (m/100cm)³ * (6.022x10^23/mol) = 99.97 molecules/cm³

    am i correct now?
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