Page:Text-book of Electrochemistry.djvu/174

 Comparison between the Results of the Osmotic and the Electric Determinations of the Degree of Dis- sociation. — Solutions of salts give what appeared to be anomalous results with respect to the lowering of the freezing point, or the vapour pressure, and the raising of the boiling point, or the osmotic pressure, inasmuch as the influence of the salts was greater than that of other sub- stances (see the preceding chapters). As mentioned on p. 59, these facts can be explained by assuming that the salt is partially dissociated. From the values obtained in any of these determinations the degree of dissociation can be calculated, and there arises the question whether the degrees found in this way are the same as those obtained from the electrical measurements. In 1887 I showed {2) that in the case of about a hundred solutions there was a good agreement between the degrees of dissociation calculated from Kaoult's results for the freezing point and from Kohlrausch's measurements of the conductivity.

The following tables contain the degrees of dissociation found by the two methods. The value obtained from the conductivity ia given under ai, and under as is the result calculated from the freezing point of a 1 per cent, solution of the substance. The following consideration shows how the latter calculation is made: If a substance in solution has the molecular weight M which corresponds with its chemical formula, then a solution which contains if grams of the substance in 100 grams of solvent should have the freezing point —18*6° (the molecular depression of the freezing point of water), and a solution containing 1 gram

in 100 grams of solvent should freeze at yr-. Instead of

this the solution freezes at — A°, which is lower than the temperature already indicated. The depression of the freezing point caused by the dissolved substance is therefore too great, indicating that the solution contains more molecules than has been assumed, i.e. a part of the dissolved molecules has been split up into smaller ones (the ions), so that the number

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