EXAMPLE $17\mathrm{.}1-1$

Diffusion of Helium

through Pyrex Glass

Calculate the steady$-$state mass flux $j_{Ay}$ of helium for the system of Fig. $17\mathrm{.}1-1$ at $500K.$ The partial pressure of helium is $1$atm at $y=0$ and zero at the upper surface of the plate. The thickness $Y$ of the pyrex plate is ${10}^{-2}mm,$ and its density ${}^{(B)}$ is $2\mathrm{.}6\frac{g}{c}{m}^3.$ The solubility and diffusivity of helium in pyrex are reported ${?}^7$ as $0\mathrm{.}0084$ volumes of gaseous helium per volume of glass, and $D_{AB}=0\mathrm{.}2{10}^{-7}c\frac{m^2}{s},$ respectively. Show that the neglect of the mass average velocity implicit in Eq$.17\mathrm{.}1-1$ is reasonable.