X-based and m-based activity coefficients in the solver
The conversion from the activity coefficient in molal basis (i.e. unsymetric) to mole fraction basis (i.e. symmetric) is the following:
On the molality basis, the activity coefficient of water is not defined (it is defined for all components except water). This is because the definition of molality, which refers everything to 1 kg of water. For all other components, we report the activity coefficients. Therefore, we report directly the activity of water in the γ–m-based column. On the mole fraction scale, there are no such exceptions because water is then treated as any other component and the γ–x-based column shows activity coefficients for all species, including water.
So,
