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Homework Help: Molecular diffusion question; fick's first law

  1. Mar 1, 2008 #1
    1. The problem statement, all variables and given/known data
    Calculate the flux of oxygen across a square centimeter membrane [2[tex]\mu[/tex], or 2x10 [tex]^{-6}[/tex]m thick if the concentration on the right hand side of the membrane is 4mL/L and on the left side is 2mL/L.
    2. Relevant equations
    fick's first law of diffusion
    Flux = dC/dx = D[C2-C1] where D is the molecular diffusion constant
    molecular diffusion constant for oxygen = 3x10^-5
    3. The attempt at a solution

    Using the first eq, I took the difference in the concentrations, and divided it by the thickness of the membrane. [4mL/L-2mL/L]/ 2[tex]\mu[/tex], which resulted in a flux of 1x10[tex]^{6}[/tex]... however, the equation did not take into account the molecular diffusion constant of oxygen, which was provided in the problem. So then I tried the second equation. 3x10[tex]^{-5}[/tex]x[4mL/L - 2mL/L]= 6x10[tex]^{-5}[/tex]. however, the second equation does not take into account the thickness of the membrane (dx). Im totally lost and have no idea what to do. please help thanks!
    Last edited: Mar 1, 2008
  2. jcsd
  3. Mar 1, 2008 #2


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    Science Advisor
    Homework Helper
    Gold Member

    You're almost there, but your flux equation looks a little off. Make sure to use the right equation and work through the problem again using the correct units, and things should work out fine.
  4. Mar 2, 2008 #3
    after doing a little research, I come to realize that the diffusion equation (Fick's first law) is actually

    Flux = -D[([tex]\Delta[/tex]C)/([tex]\Delta[/tex]x]
    where D is the diffusion coefficient;
    where C is the concentration
    where [tex]\Delta[/tex]x is the thickness of the membrane

    therefore, -3x10^-5 [(2mL/L)/(2x10^-6m)]= -30 [mol]/[m^2 sec]
    Last edited: Mar 2, 2008
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