Dissociation constants, acetic acid strong electrolytes

Conductance measurements also have been used for the estimation of dissociation constants of weak electrolytes. If we use acetic acid as an example, we find that the equivalent conductance A shows a strong dependence on concentration, as illustrated in Figure 20.2. The rapid decline in A with increasing concentration is largely from a decrease in the fraction of dissociated molecules. 

PK. A measurement of the complete ness of an incomplete chemical reaction. It is defined as the negative logarithm ito the base 101 of the equilibrium constant K for the reaction in question. The pA is most frequently used to express the extent of dissociation or the strength of weak acids, particularly fatty adds, amino adds, and also complex ions, or similar substances. The weaker an electrolyte, the larger its pA. Thus, at 25°C for sulfuric add (strong acid), pK is about -3,0 acetic acid (weak acid), pK = 4.76 bone acid (very weak acid), pA = 9.24. In a solution of a weak acid, if the concentration of undissociated acid is equal to the concentration of the anion of the acid, the pAr will be equal to the pH. 

Hence, in water, which has a very high dielectric constant, the so-called strong electrolytes include all salts and strong acids and bases. The weak electrolytes are almost all organic acids and bases. However, this distinction depends on the characteristics of the solvent. Some solvents are more acidic than water (e.g., formic and acetic acid), while the others are more basic (e.g., ethylenediamine). Besides, the permittivity of organic solvents is generally smaller than the permittivity of water. So, the dissolution of a potential electrolyte in a nonaqueous medium proceeds in two steps ionization and dissociation. 

Note the strong dependence on the charge number. As shown elsewhere, the equilibrium constant of a 1 1 weak electrolyte like acetic acid is increased by an electric field of 100 kV cm to about 14%, that for a 2 2 electrolyte like MgS04 to about 110%. ° Compared to simple dipolar equilibria of small molecules where electric-field-induced changes in K are very small, we see that the dissociation step of simple ion pairs is associated... 

In the case of weak electrolytes, Aq may be determined either from a knowledge of the dissociation constant of the electrolyte or by making use of the Kohlrausch independent migration relation. For example, Aq for acetic acid may be calculated from experimentally determined values of Aq for hydrochloric acid, sodium acetate and sodium chloride - all strong electrolytes. From the Kohlrausch relation we may write... 

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