Molecular diffusion question; fick's first law

[aquabum619]

Homework Statement

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.

Homework 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

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). I am totally lost and have no idea what to do. please help thanks!

 

[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.

 

[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]