I'm confused as to what full ionization mean? I have the mean molecular weight of electrons and ions and I have to calculate the ideal gas pressure for a star. Do I have to add them up as usual or do I use only the mean molecular weight of ions?
Full ionization means that the electrons are no longer bound to the nuclei: the electrons and the nuclei behave as independent particles. I think this means you can treat them as a gas - the same way that you would treat a mixture of gases.