Nitrogen from a gaseous phase is to be diffused into pure iron at $700C,$ If the surface concentration is maintained at $0\mathrm{.}22$ wt $\%N,$ what

will be the concentration $($in weight percent$)3\mathrm{.}2$ mm from the surface after $7\mathrm{.}2$ h $?$ The diffusion coefficient for nitrogen in iron at

$700C$ is $2\mathrm{.}6{10}^{10}\frac{m^2}{s}$