A first order reaction with a rate constant of k takes place in a tubular reactor with a length of L and a diameter of D$.$ The reactant A is fed into the reactor in a solution with an inlet concentration of CAo. The diffusivity of A in the liquid phase is given as DA$.$ L$/$D ratio of the reactor is extremely grater than $1.$ By performing appropriate assumptions make microscopic mass balance around the system when the axial diffusion is neglected. Then solve the differantial equation for obtaining the reactant concentration in the axial direction.