Fraction dissociated

Figure 2.12 also shows a curve representing the relation between the values of log (cKa) and the degree of ion association (a). This curve shows that, for 10-2 M KC1 in EtOH (logKA = 2), log (cKA)=0 and 38% of KC1 exists as ion-pairs. For 0.1 M KC1 in acetic acid (log KA=6.2), 99.9% of KC1 exists as ion-pairs the fraction dissociated to free ions is only 0.1%. Usually, in solvents with er<10, the fraction of electrolyte that is dissociated into ions is small except in very dilute solutions. Conversely, in solvents with er>40, the fraction of electrolyte that forms ion-pairs is small except in fairly concentrated solutions.11 ... 

Reeves (1977) has used the temperature dependent dissociation of the imidazole side-chain to develop the alphastat model, which focuses on conservation of alpha-imidazole (ajmjd), the fractional dissociation of histidyl imidazole groups ... 

Fractionation dissociation extraction involves both the chemical reaction and the fractionation scheme for the separation of components by their difference in dissociation constants as described by Colby [in Hanson (ed.). Recent Advances in Liquid-Liquid Extraction, Pergamon, New York, 1971, chap. 4]. 

Let us calculate some fractional dissociations of H2CO3. Its dissociation reaction is as follows ... 

Now, proceed to calculate the fractional dissociation of the hydroxide ion from sodium carbonate. Begin by assuming a concentration for the carbonate of 0.1 M. This will then produce, also, 0.1 M of the carbonate ion. By its subsequent reaction with water, however, its concentration at equilibrium will be smaller. Let x be the concentrations of HCO, and OH" at equilibrium. Then the concentration of CO," is 0.1 - X (at equilibrium). Thus,... 

FIGURE 4-2 Left, relation between the observed formation constant for the salt tetia n-butylammonium picrate and the dielectric constant of the solvent. From Inami, Bodenseh, and RamseyJ ) Right, relation between fraction dissociated and dielectric constant calculated from data in the left part of the figure. 

Direct calculation of extinction coefficients (and hence oscillator strengths) requires a knowledge of both intensity of absorption and concentration of absorbing species. For the transition described above, concentrations of OH were calculated from the known fractional dissociation of water at elevated temperatures. In a few other instances, it may be possible to estimate the concentration of the intermediate from chemical considerations. Thus, Lipscomb et alP were able to calculate the extinction coefficient at A = 2577 A of the radical CIO from flash photolysis studies of chlorine dioidde. It was shown that the disappearance of CIO obeyed a second order rate law, so that... 

It decreases with increasing ionic strength due to the corresponding decrease in and increase in jjp. As a result, there is a slow increase in a, the fraction dissociated, at higher concentrations. 

The expression for formation of the weak acid, association, can also be written in terms of the fraction dissociated, a. 

The solubility of SO2 in water is not quite proportional to the pressure, because the H2SO3 that forms partially dissociates to and HSO3, and the fraction dissociation decreases with increasing concentration. Henry s law applies to the undissociated H2SO3, and the total SO2 dissolved is the sum of the HjSOj and the HSO3. (The SOj" formed in water is negligible.) A few points for the total SOj dissolved at 80°F are given below. 

Suppose that n mol of the electrolyte are present in V dm, and that the fraction dissociated is a. The fraction not dissociated is 1 — a. The amounts of the three species present at equilibrium, and the corresponding concentrations, are therefore... 

This equilibrium can be easily studied in the laboratory through a measurement of the vapor density of the equilibrium mixture. In the following formulation the various quantities are listed in columns under the formulas of the compounds in the balanced chemical equation. Let rf be the initial number of moles of N2O4, the equilibrium advancement, and the fraction dissociated at equilibrium = Jn°. 

The fraction dissociated at 20 C in 0.05 mole fraction solution in cyclohexane. The average degree of aggregation in freezing cyclohexane. 

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