Page:Text-book of Electrochemistry.djvu/80

 Molecular Rise in Boiling Point in Watek.

rfr(calculated) = 6-2.

Attention may here be called to the following results. The dissimilar colours of iodine in benzene, ether, and acetic acid (brown), and in carbon disulphide (violet), were previously- attributed to different molecular magnitudes of the iodine. Beckmann's results (20), however, indicate that in all these solvents the molecular weight of the dissolved iodine corresponds with the formula la (254), and is the same as that of iodine vapour at low temperatures. Phosphorus has the same molecular weight (P4 = 124) when dissolved in carbon disulphide as in the gaseous state. For sulphur dissolved in carbon disulphide, Beckmann found the molecular weight, 256, corresponding with the formula Ss, which is the same as that obtained by Biltz and V. Meyer (2 J) for sulphur vapour.

Comparison between the Various Methods for the Determinatioii of Molecular Weights. — It must now be clear that the several methods for determining the mole- cular weight, by measurement of the osmotic pressure, of the depression of vapour pressure and freezing point, and of the rise of boiling point, lead to the scmie result, provided that in all cases the temperature is the same. The results of Bonders and Hamburger (^2) show that temperature has only a small influence on the relative values of the osmotic pressure, and this is further proved by a comparison of the values obtained by Tammann for the lowering of the vapour pressure at 100° with those found by Dieterici {2J) at 0°. This result is also required by theory, as we shall see later, in those cases where no heat change takes place on dilution of the solution, a condition which is very nearly fulfilled with dilute solutions. Consequently we may assume that when results are obtained which are at variance with the theory,

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