Solving the Concentration Gradient: A Calculation Challenge

[tkuehl]

I have a problem with calculating the concentration gradiant. Here is the question and the solution from the solution manual and the numbers don't add up.

A 1-mm sheet of FCC iron is used to contain nitrogen in a heat exchanger at 1200℃. The concentration of N at one surface is 0.04 atomic percent and the concentration at the second surface is 0.005 atomic percent. Determine the flux of nitrogen through the foil in atoms/cm2−s.

solution
(a) Ac/Ax = [.00005 - .0004]*4 atoms/cell(3.589E-8)^3
--------------------------------------------
.1cm
=-3.03 x 1020 N atoms/cm3-cm

(b) J=-D(Ac/Ax) = -0.0034 exp[-34,600/(1.987)(1473)](-3.03 x 1020)
= 7.57 x 1012 N atoms/cm2-s

I would like to know how they get the answer in part a.

 

[tkuehl]

I figured it out. the volume of the cell needs to be in the denominator. Our professor takes everything out of the solutions manual and never checks it.