Although the essential features of binary mass transfer are understood fairly well, there is a rather startling dearth of knowledge of multicomponent mass transfer. Therefore, the general equations that describe multicomponent mass transfer may not be obtained easily. Furthermore, the equations for diffusion of multicomponent mixtures are quite complicated. Using binary diffusivity values and assuming that all components are stagnant except one of the components, write an expression for the diffusivity of a multicomponent system. Define carefully all nomenclature used. [5]