Switching from concentrations to molalities

Up until now, we have chosen to quantify the composition of solutions in concentrations (molarities) expressed in moles/liter. Very often, users of Debye and HuckeTs model use molality values, which are expressed in moles per kilogram of solvent. Between these two values, if the solution is sufficiently dilute (which is the case in the domain of validity of Debye and HuckeTs model) to enable us to treat the volume of the solution and that of the solvent as one and the same thing, we can write  

The coefficient 1000 stems from the fact that the molality is given in moles/kg and the molarity in moles/1, rather than in moles/m.  

If we work on the basis of the second expression of the electrical excess Gibbs energy given by relation [4.76], using expression [4.102], we obtain  

we define an ionic strength in terms of molality, which is expressed, similarly to definition [A.2.49], by  

the electrical excess Gibbs energy can be written  

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