The mean$-$free path between collisions of gas molecules depends on

pressure. At what pressure will the mean$-$free path of spherical

molecules $(0\mathrm{.}3-$nm in diameter$)$ be equal to $20$ nm$?$ Note that this is

the pressure at which bulk diffusion and Knudsen diffusion are equally

important when the diameter of catalyst pores is $20$ nm$.$ Perform the

calculation at $500$ K$.$ Make other reasonable assumptions that you

deem necessary $($e$.$g$.,$ Molecular weight is $30$ g$/$mol$,$ etc.$).$