In an O$2-$N$2$ gas mixture at $1\mathrm{.}01325$ bar and $20$oC the concentration of O$2$ at two plans $2$mm apart are $25\%$ and $15\%$ volume respectively. Calculate the rate of diffusion of O$2$ expressed as kgmoles of oxygen$/$m$2$ for the case where N$2$ is non diffusing, diffusivity of oxygen is $0\mathrm{.}181$x$10-4$ m$2/$s$.$