You have been tasked with assessing the diffusion coefficient $($D$)$ of a new chemical. To do so$,$ you experiment with a tank that is $40cm$ depth. You spread $100g$ of the chemical uniformly over the tank$\text{'}$s surface at a depth of $20cm$ and track its development over a period. The chart below illustrates the vertical profiles of chemical concentration in the tank $($the $x-$axis displays depth and the y$-$axis displays concentrations$).$

a$)$ Using the graph, calculate the diffusion coefficient of the chemical, D$.(15$ points$)$

b$)$ Determine when the tank$\text{'}$s borders will have an impact on the chemical's vertical distribution