4. You have been tasked with assessing the diffusion coefficient (D) of a new chemical. To do so, you experiment with a tank that is 40 cm depth. You spread 100g of the chemical uniformly over the tank's surface at a depth of 20 cm 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.

b) Determine when the tank's borders will have an impact on the chemical's vertical distribution.