Nitrogen is diffused into pure iron at $675C,$ with a constant surface concentration of $0\mathrm{.}1wt\%.$

$($a$)$ How long should the treatment last such that the concentration $3mm$ from the surface is

$0\mathrm{.}03wt\%N?D=2\mathrm{.}8{10}^{-11}\frac{m^2}{s}.$

$($b$)$ What is the total amount of nitrogen diffused into the iron at this time, per unit area? Give your

answer in units of $mo\frac{l}{m^2}.$