A diffusion couple, made by welding a thin one‐centimeter square slab of pure metal A, to a similar slab of pure metal B, was given a diffusion anneal at an elevated temperature and then cooled to room temperature. On chemically analyzing successive layers of the specimen, cut parallel to the weld interface, it was observed that at one position, over a distance of 5000 nm, the atom fraction of metal A, N _A changed from 0.30 to 0.35. Assume that the number of the atoms per m ³ of both pure metals is 9 x 10 ²⁸ . First determine the concentration gradient dn _A /dx. Then if the diffusion coefficient, at the point in question and annealing temperature, was 2 x 10 ^-14 m ² /s, determine the number of A atoms per second that would pass through the cross‐section at the annealing temperature.