K Equal to a Solubility

At this point the expression is perfectly general, valid for any conditions, and K is calculable from equation (13.8) if we know the standard state free energies of the three constituents. If the system we are considering is simply quartz in water at T and P, and if we define our standard states to be pure quartz and pure water at T and P, and ideal one molal H4Si04 at T and P, then both quartz and water have activities of 1.0, and 

Albite-Nepheline Example Next consider the reaction 

Using the same standard states, that is, the pure minerals and water at T and P, the equilibrium constant expression for this reaction also reduces to 

Let s suppose that a measurement of quartz solubility has been used to obtain the free energy of formation (standard or apparent) of H4Si04 in the ideal one molal standard state. This number can then be used (with A/G° terms for the minerals) to calculate the equilibrium constant of the albite-nepheline reaction (equation (13.11)), giving the equilibrium silica concentration in a solution that may never have been experimentally determined, or perhaps never existed, and in which quartz is not stable. Thus knowing the solubility of quartz, one could in a similar way calculate the silica concentration in fluids in contact with a variety of mineral assemblages. 

While we are on the subject, let s look at the same situation in a different way, as another illustration of the use of activities. If we write 

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