Stoichiometric, mean ionic

This has always been an important, though very indirect, source of hydration numbers. Colligative properties and emfs give high precision experimental data from which the activity of the solute can be calculated and stoichiometric mean ionic activity coefficients found. If it is assumed that solvent molecules are bound to the ions, a relation between... 

Fig. 17.4. Stoichiometric mean ionic activity of HCl (a ,Hci) versus molality of HCl. Crosses are data points from Table 17.2 dotted line is Henry s Law, having an activity of 1.0 at 1.0 molal HCl. The stoichiometric mean ionic activity coefficient at muci =0J sxy/xz = 0.772.

J. C. Poirier, Thermodynamic functions from Mayer s theory of ionic solutions. II. The stoichiometric mean ionic molar activity coefficient, /. Chem. Phys. 21,972 (1953). 

The Debye-Hiickel equation in terms of stoichiometric mean ionic activity coefficients is (where yn indicates it is a Henryan activity coefficient)... 

In the following example, data from Archer (1992) on the stoichiometric mean ionic activity of NaCl at 25 are fitted to Equation (15.28), with a and B as unknowns. 

Figure 15.4 Curves labeled k= , etc., are calculated from Equation (15.22). Diamond symbols are stoichiometric mean ionic activity coefficients for NaCl at 25°C from the EoS of Archer (1992) as a function of (stoichiometric) ionic strength. The curve through the diamond symbols is calculated using Equation (15.28)...

Ideally one would wish to measure the activities of all the reacting species in equation (4) but it is not possible to determine activities of single ionic species (at best, mean ionic activities may be determined in some cases) and the determination of activities of uncharged species is fraught with difficulties. However, modem experimental methods (see Section 3) can readily be applied to the determination of concentrations of at least some of the species concerned, leading to stoichiometric stability constants defined by... 

The activity of each ionic species may be represented as the product of its molality and its (stoichiometric) activity coefficient hence is equal to my. and oci to 2my, where m is the molality of the zinc chloride solution in the cell. By equation (39.9), 7+7I is equal to y , where y is the mean ionic activity coefficient hence equation (45.16) becomes... 

Therefore other classes of activities and activity coefficients are defined ( mean ionic activities and mean ionic activity coefficients, without the stoichiometric ), which are related to the stoichiometric quantities by the degree of dissociation, a, where... 

Recall that y is the mean ionic activity coefficient of a strong electrolyte, or the stoichiometric activity coefficient of an electrolyte that does not dissociate completely. 

Thus, this is an alternative method for the estimation of the mean ion radius in mixed-valence oxides or in non-stoichiometric oxides, using values of metal ion radii in various oxides of the same metal. The above method allows for the prediction of a mean ionic radius in oxides having so much deformed structure that it is difficult to determine the coordination number of metal ions or this number changes with the oxidation state. 

The label on a solution refers to the stoichiometric or weight composition, not the concentration of any particular component in an ionic equilibrium. In other words, 0.0100 M NH3 means that the solution was produced from 0.0100 mol ammonia and the necessary water to make a liter of solution. It does not mean that the concentration of the un-ionized ammonia in the solution, [NH3], is 0.0100. 

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