Concentrated Chemical Species

Hello All,

I am in trouble defining the constraint for the dependent variables (mass fractions) in Concentrated Chemical Species physics.
I have the mass fraction of Species A to E and one inert (total 6 chemical species): wA, wB, wC, wD, wE and wI (inert). There are two reactions:
A + B < = > C + D .......(1)
C + B < = > D + E .......(2)
Now, the reaction (2) is in equilibrium with reaction (1) and I have the rate law for the reaction (1), so from equilibrium I can calculate the concentration of E in reaction (2). In Comsol I don't need to define wE (mass fraction of E), as I can calculate its concentration from the equilibrium but it is only possible when I satisfy the condition wA+wB+wC+wD+wE+wI = 1. So somehow I need to let Comsol know that there is another chemical component E which is in equilibrium with the remaining, and constraint is sum of all species mass fractions = 1.
let me know how to deal with such situation in Comsol.

Thanks all
Mayur