b$)$ This species is diffusing through a Cu foil with thickness $0\mathrm{.}12mm.$ At $1400C,$ what is the diffusion flux through the foil, if the species concentration is $0\mathrm{.}5mo\frac{l}{c}{m}^{{?}^{{?}^{?}}}3$ on one side of the foil and $0$ on the other?

c$)$ Now, instead of a thin foil, you have a long, pure $($no initial concentration of this impurity species$)$ Cu rod. If the source concentration is still $0\mathrm{.}5mo\frac{l}{c}{m}^{{?}^{{?}^{?}}}3$ and diffusion is still occurring at $1400C,$ how long in days will it take for the concentration $1cm$ into the rod to reach half the source concentration?

The solution to Fick's $2$nd Law for a point source diffusing into a semi$-$infinite solid is: $\frac{C(x,t)-C_0}{C_8-C_0}=1-$erf$(\frac{x}{2\sqrt[\phantom{2}]{Dt}})$

d$)$ Based on your answer to $($c$),$ is the semi$-$infinite assumption reasonable? Why or why not?

$\backslash$table$[[z,$erf$(z),z,$erf$(z),z,$erf$(z)$