Hello, I am currently using COMSOL to do multi-component unsteady state 1D diffusion modeling (symmetry). The chemical mixture consists of 5 different chemical components which are modeled using classical Fick's second law of diffusion. To ensure mass balance based on a simplistic approach the mole fraction of the fifth chemical at any point in time will be 1- summation of the mole fraction of the 4 chemicals, since the total mole fraction of the multi-component mixture must be unity. I have the following questions 1) Do I use the transient diffusion analysis for my modeling or the Convection-Diffusion analysis (PDE mode) or any other analysis (PDE modes)? Currently I am using the Transient diffusion mass transfer mode. 2) Most importantly how do I incorporate the mass balance (mole fraction chemical 5 = 1- sum(mole fraction of chemicals 1 thro' 4)) in the analysis ? Thanks for the help in advance.