How to find the molar volume of 1 gas in a 2 gas system using PV_m=ZRT?
If you want to get the partial molar volume of a species in a non-ideal gas mixture, you can treat it as an ideal solution.  In this approximation, the partial molar volume of the species is equal to the molar volume of the pure species at the same temperature and pressure as the solution.  Another method is to evaluate the molar weighted averages of the critical pressures, critical temperatures, and ascentric factors for the various species in the mixture, and then treating the mixture as a pure gas.  You then take the partial derivative of the volume with respect to the number of moles of the species under consideration.  I would try it both ways to see how the results compare.