# X-based and m-based activity coefficients in the solver

There are two types to activity coefficients according to they way they are defined: The activity coefficient in molal basis (i.e. unsymetric) and the activity coefficient in the mole fraction basis (i.e. symmetric).

To convert the activity coefficient from molal basis to the the mole fraction basis you can use the following equation:

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.

However, the activity coefficients in both the molal and mole fraction base should be the same: