The stagnant liquid $B$ in a container that is $10cm$ high $(L10cm)$ is exposed in air $(A)$ at time$=0.$

The material balance of $A$ is equal to:

$\frac{del{c}_A}{\text{d}\text{e}\text{l}\text{t}}=D_{AB}\frac{de{l}^2{c}_A}{del{z}^2}$

Initially the concentration of A into the liquid is equal to zero,

The concentration at the interface is equal to $C_{AO}{?}_{{?}_{?}}$

a$.$ draw your system,

b$.$ discretise the equation $(1)$ for a generic node.

c$.$ define the initial condition and the boundary condition

d$.$ discretise c$.$

e$.$ explain the methodology to calculate the time for reaching the concentration CA$0$ at z$-9$

cm$.$