Stoichiometric and Effective Ionic Strength

When the total concentrations of dissolved species are used to compute ionic strength through its definition 

Compute the total or stoichiometric and effective ionic strengths, and corresponding total- and free-ion activity coefficients of Ca and SO in pure water saturated with gypsum at 25 C. It is given that the solubility product of gypsum, and that at gypsum saturation 

The effective ionic-strength approach assumes the existence of ion pairs, thus the mass-balance equations are 

Calcium and sulfate ions form the CaSO ion pair, with the association constant expression 

Note that 1 is about 50% greater than 7. Calcium and sulfate ion activities must be the same, whether computed assuming effective or stoichiometric /. Because y = a/m , we find that for /j 

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