A)The number of particles is given by n=-D (n2-n1/x2-x1) are crossing a unit area perpendicular to x axis in unit time, n1 and n2 are the number of particles per unit volume for the values of x meant to be x1 and x2. What is the dimensional formula of Diffusion constant D?(Please explain the working.) b) Find the dimensional formula of 1*e^2/4*pi*epsilon*h*e. (Please also explain what does e stand for.)



You solve this like any other equation. 
According to the text you have the following units. 

[ ] means dimension 

[n] = [L]^(-2) * [T]^(-1) 
[n2-n1] = [L]^(-3) 
[x2-x1] = [L] 

[n] = [d]*[n2-n1]/[x2-x1] 

put the above values in: 

[L]^(-2) * [T]^(-1) = [d] * [L]^(-3) / [L] 

[d] = [L]^(-2) * [T]^(-1) * [L] / [L]^(-3) 

[d] = [L]^(-2+1+3) * [T]^(-1) 

[d] = [L]^(2) * [T]^(-1)
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