Certain gas "A" is dissolved in a liquid "B" contained in a beaker. As "A" diffuses, it undergoes an irreversible zero-order chemical reaction of the form: A + B C. The formation of C does not interfere with diffusion. Upon reaching a certain length "S" from the interface, the species "A" disappears completely. 

Find an expression for the average molar concentration of "A" at the diffusion length "S".