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��42 VAPOUR PRESSURE OF SOLUTIONS. chap.
of the dissolved substance, and is conditioned solely by the number of dissolved and solvent molecules.
This law was first established by Baoult {3) from purely experimental results ; the connection with osmotic pressure was later shown by van't HoiBf {Jfi), gram-molecules of a solvent, the relative lowering of the vapour pressure should amount to -fo o- Eaoult carried out a large number of experiments on this subject.
The following table gives the mean values for a series of solvents from results obtained ^vith various dissolved substances : —
Carbon tetrachloride . . O'OIOS
��Ether % . 0-009G
��It has occasionally been assumed that by means of the above formula the molecular weight of the solvent could be calculated as that of the dissolved substance can be, since the relationship between the number of molecules of dissolved substance and of the solvent apparently occurs in the equation. If we regard the deduction more closely, however, we see that we have assumed the same molecular weight for the. solvent in the liquid and in the vapour state. The molecular weight for the solvent in the vapour state must be that deduced from the vapour density, for otherwise A vogadro's law, which is used in developing the formula, would not be applicable. WTien Baoult experimented \vith acetic add, the molecular weight of which, according to the chemical formula, is 60, but from the vapour density is 972 (= 1*62 x 60), he obtained for the relative lowering of the vapour pressure a number which was nearly 1*62 (exactly 1'63) times greater than that calculated, assuming 60 for the molecular weight. This ap- parent deviation is, therefore, in complete accord with the law.
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