How do you derive it? I'm looking for the form J = -D(dC/dX) From the image, I am assuming left side is of higher concentration. N represents the number of molecules. My work: Mass balance Net molecules in from concentration gradient = Net = N(x) - N(x+Delta X) Net concentration in = molecule/volume = [N(x) - N(x+delta x)] / (A * Delta X) = dC And then I'm stuck! I probably went about this totally wrong.