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

1. Mar 1, 2008

aquabum619

1. The problem statement, all variables and given/known data
Calculate the flux of oxygen across a square centimeter membrane [2$$\mu$$, or 2x10 $$^{-6}$$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$$\mu$$, which resulted in a flux of 1x10$$^{6}$$... 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$$^{-5}$$x[4mL/L - 2mL/L]= 6x10$$^{-5}$$. 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. Mar 1, 2008

Mapes

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.

3. Mar 2, 2008

aquabum619

after doing a little research, I come to realize that the diffusion equation (Fick's first law) is actually

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

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

Last edited: Mar 2, 2008