Nitrogen from a gaseous phase is to be diffused into pure iron at $700C.$ If the surface concentration is maintained at $0\mathrm{.}12$ wt $\%N,$ what

will be the concentration $($in weight percent$)3\mathrm{.}6mm$ from the surface after $5\mathrm{.}8h?$ The diffusion coefficient for nitrogen in iron at

$700C$ is $4\mathrm{.}0{10}^{-10}\frac{m^2}{s}.$