Nitrogen from a gaseous phase is to be diffused into pure iron $($BCC$)$ at $721\backslash$deg C$.$ If the surface concentration is maintained at $0\mathrm{.}10$ wt$\%$ N$,$ what will the concentration $1$ mm from the surface be $($in wt$\%$ N$)$ after $7\mathrm{.}71$ hrs$?$ Use the table below for error function values as needed.