Consider a long duct that spans from $-$to $+$o$.$ At x $=0,$ two gases $($hydrogen and nitrogen$)$ are separated by a membrane. H$2$ spans from x $=-$ to x $=0$ and N$2$ spans from x $=0$ to x $=+$o$.$ Suddenly, the membrane is removed and the gases diffuse into one another. It was derived in the class that the mass fraction of H$2$ can be estimated from: Yu $($n$)=[1$ erf$($n$)],$ m $(1)$ where n $=$ x$/$V$4$Dt$.$ t is time and D is the bi$-$molecular diffusivity of hydrogen and nitrogen. Assume D $=5$ x $10-5.$ a$.$ Plot variation of H$$ mass fraction with x at t $=10,102,103,104$ seconds and t $=0.$ b$.$ How long does it take for the hydrogen mass fraction at x $=-0\mathrm{.}5$ m to decrease from about unity to $0\mathrm{.}8.$