How far out into the solution do we go

It was previously said that there are two concentration boundary values, not determined by diffusion Cq at x 0 and Cj at infinity. According to the mathematics of diffusion, treating the diffusing solute as 

Let us look at some actual numbers arising from this. An experiment runs for time t or for T = 0..1. We decide on a simulation using 1000 time steps, and that X should be 0.4 then, using Eq. 3.47, 

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