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Homework Help: Basic Kinetic theory: How many molecules are there?

  1. Nov 28, 2015 #1
    1. The problem statement, all variables and given/known data
    A closed vessel contains liquid water at equilibrium with its vapor at 100C and 1 atm. 1 gram of water vapor occupies 1670cm^3. Hvap = 2250Jg^-1. How many molecules are there per cm^3 of vapor?How many vapor molecules strike each cm^2 of liquid surface per second?

    2. Relevant equations
    PV = NKT
    Q = lm

    3. The attempt at a solution
    N = PV/KT = 3.3E26 molecules of vapor
    I used P = 1.013 E5 N m^-2
    V = 16.7m^3
    T = 373
    Not sure how to get how much water there is.
    Answers are: 2E19 molecules cm^-2 ; 3.3E23 impacts per second
  2. jcsd
  3. Nov 28, 2015 #2


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    Science Advisor

    How many water molecules are there in 1 gm of H2O? You are given the volume of 1 gm, so given these two, you should be able to calculate the number of molecules in 1 cm^3.
  4. Nov 28, 2015 #3


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    Staff Emeritus
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    Homework Helper

    BTW, 1670 cm3 ≠ 16.7 m3. Check your unit conversions again, since 1 m3 ≠ 100 cm3. :frown:
  5. Nov 28, 2015 #4
    Wow. Thanks
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