Abnormal molar weights

Abnormal Molar Weights.— In the case of a number of substances (very commonly in the case of organic acids and hydroxy-compounds in benzene), it is found that the molar weight determined by the cryoscopic method is greater than that calculated from the usual chemical formula of the substance, by an amount exceeding the experimental error. We are therefore led to the assumption that these substances associate in solution, i.e, two or more molecules combine to 

Experiment.—In the manner Just described, determiru the apparent Molar Weight of Benzoic Add in Benzene, and from the numbers obtained calculate the Degree of Assodation, assuming that two single molecules combine to form one compound molecule. 

The degree of association can be calculated in the following manner If x represents the degree of association, or the fraction of the total number of molecules which combine to form larger molecules, and if n represent the complexity of the new molecules, then of each mole of substance taken there will 

Consequently, instead of there being i mole there will be only i—x+ oii—x T—. In other words, the number of 

But the depression of the freezing-point is proportioned to the number of moles (in a given volume) hence, if d, represent the depression calculated on the assumption of no association, and d the depression actually obtained— 

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The molar volume of a substance in solution is given by Vm= nC+ where the solution co-volume is 12 4 ml. at 15°. The atomic and structural constants are as above, except for those in brackets (for solutions), but values for metals were also calculated, and if the salt is ionised, 13 5 is subtracted from UnC. Abnormal values of are again used to calculate molar weights. The value... 

Associating and Dissociating Substances.—In the case of certain substances it is found that the molar weight, calculated from the vapour density, has a value sometimes greater and sometimes less than that corresponding with the formula which, on other grounds, must be assigned to the substance and it is also found that in those cases, the vapour density is not independent of, but alters with, the temperature. These cases of abnormal vapour densities, as they were termed, are accounted for by the assumption of association or of dissociation of the molecules of the substance in the vapour state and from the values of the density obtained one can... 

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