A sheet of steel $3\mathrm{.}5$ mm thick has oxygen atmospheres on both sides at $850$C and is permitted to achieve a steady$-$state diffusion condition. The diffusion coefficient for oxygen in steel at this temperature is $5$x$10^-10$ m$2/$s $,$ and the diffusion flux is found to be $2\mathrm{.}3$x$10^-8$ kg$/$m$2$s$.$ Also $,$ it is known that the concentration of oxygen in the steel at the high$-$pressure surface is $3,7$ kg$/$m$3.$ What will the concentration at $1$ mm far into the sheet from this high$-$pressure side? Assume a linear concentration profile.