It can be advantageous to describe the aqueous phase in terms of molality instead of concentration. The molality of a solution does not change with , since it refers to the mass of the solvent. In contrast, the concentration  does change with , since the density of a solution and thus its volume are temperature-dependent. Defining the aqueous-phase composition via molality has the advantage that any temperature dependence of the Henry's law constant is a true solubility phenomenon and not introduced indirectly via a density change of the solution. Using molality, the Henry solubility can be defined as
Here  is used as the symbol for molality (instead of ) to avoid confusion with the symbol  for mass. The SI unit for  is mol/(kg·Pa). There is no simple way to calculate  from , since the conversion between concentration  and molality  involves all solutes of a solution. For a solution with a total of  solutes with indices , the conversion is:
where  is the density of the solution, and  are the molar masses. Here  is identical to one of the  in the denominator. If there is only one solute, the equation simplifies to
Henry's law is only valid for dilute solutions where  and . In this case the conversion reduces further to
and thus