Chemical Potential and Gibbs Energy of Formation

There is a fundamental problem in thermodynamics we do not know the absolute values of some thermodynamic functions. For example, the absolute values of such properties like internal energy, enthalpy, and Gibbs energy of a chemical are not known. The absolute values of the standard chemical potentials and are 

The equations for the Gibbs energy of formation of water and the ith electrolyte can be written as follows  

Note that fugacity of a pure substance can be calculated using the corresponding equation of state [4] and a gas component fugacity can be calculated similarly to activity, that is, as a product of the component partial pressure and the fugacity coefficient. Details of such calculations can be found in any chemical thermodynamics and some physical chemistry books [4]. 

Using data from Ref. [2], A(G and A(G can be calculated for the NaCl(aq) saturated solution. Employing a short extrapolation for calculating y of the saturated NaCl(aq) solution, the following values can be obtained A/ = -237.83 kJ mol and AjG, = -384.38 kJ mol , which can be checked for their validity because this is a three-phase system and all phases are in equilibrium. From [Chapter 10, Table 10.3], the Gibbs energy of formation of solid NaCl, AfG aCKs) = -384.10 kJ mol .  

The mean activity coefficient of NaCl(aq) in the saturated solutions can be found as 0.953 by the extrapolation of the data from [Chapter 10, Table 10.17], 

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