If J-J^ = "^ , the two equations are similar, only a is
replaced by ai. Therefore a is equal to ai, and in the same way it can be shown that/3 is equal to /3i. This relationship, developed for weak acids, can also be applied to strong electrolytes. Solutions which on mixing do not change their dissociations (and consequently their other properties) are extremely important, and are called isohydric solutions. The conductivity of a mixed solution can thus be easily arrived at ; we have only to think of the solvent water so distributed between the dissolved substances that the solu- tions formed are isohydric, i.e. contain the same number of gram-ions per litre. If the substances contain a common ion, no change in dissociation takes place on mixing, and the conductivity can be calculated as the sum of the con- ductivities of the several ions.
If two salts, as, for instance, potassium chloride and sodium nitrate, have not a common ion, there are formed in the mixed solution the other two possible salts, in this case potassium nitrate and sodium chloride. It can easily be proved that for the four salts KCl, KNOs, NaCl, and NaNOa, present in the quantities Mi, M^, Mq, and M^, and whose degrees of dissociation are au 02, as, and 04, there exists the following relationship : —
aiMi X OiM^ = aa-J/2 X 033/3.
Precipitation. — The connection just mentioned is valid for homogeneous systems, but it must be slightly modified when one of the reacting substances is difficultly soluble. Silver acetate in water is a case in point. The saturated solution of this substance at 186° is 00593-normal, and the difficult solubility is due to the fact that water can dissolve only little of the undissociated part of this salt. The dissolved quantity of the salt may as a close approxima- tion be assumed to be constant; let it be represented by ^(AgCHsCOO). If a foreign substance be added to the solution, which substance on dissolving gives (silver ions or)
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