[Opaque]

I am not certain what kind of multicomponent fluid you are trying to model.

Hopefully, you are not bypassing the ANSYS CFX chemistry/reacting modeling capabilities.

Here is an idea for you to try, at you own risk:

1 - Create the multicomponent fluid, say two materials A, and B, named "mixtureAB"
2 - Select "mixtureAB" for the Fluid definition for the domains you have in your setup
3 - Define the model you want to use for each material within the mixture as "Algebraic Equation" except for one. That material will be your ballast/constraint
4 - Input the expression that defines the Mass Fraction as a function of Temperature as you described in your original post. Be mindful and careful with the units/dimensions

Run the simulation. Hopefully those expressions for Mass Fraction make sense, and the solution will converge.