A gas$-$oil feedstock is irreversibly and very rapidly cracked on a heated metal plate in an experimental reactor. The cracking reduces the molecular weight by an average factor of three. Calculate the ratc of this process, assuming that the gas oil diffuscs through a thin unstirred film of thickness $l$ near the plate. Note that the reagent must be constantly diffusing against product moving away from the plate. Compare this rate with that for diffusion through a thin film and with evaporation through a stagnant solvent.