Agaseous chemical species (a) is transferred and reacted in equimolecular counterdiffusion in a catalytic particle with cylindrical geometry of radius r. the chemical reaction is elementary and first order with the expression, where k 'is the kinetic coefficient with the units h-1, ca, ra are given in mol / m3 and mol / m3-h, respectively. derive the profile of concentrations and the molar flux proposing a balance of mass transfer.