THE MOLECULAR FORMULE OF LIQJJIDS. 167 XVL-Tize Molecular Formulce of some Liqzcicls, as determined by their Molecular Surface Energy. By EYILY Asrox, B.Sc., and WLLLIAM RAMSAY, Ph.D., F.R.S. IN con tinnation of researches published in these Transactions (18933, 63, lOS9), we proceed to bring under the notice of the Society ex- periments on phenol,* bromine, nitric and snlphnric acids, and phosphorus. It has been our object to select liquids which, from their behaviour or from their analogy with others already investi- gated, promised to show a higher molecular weight than that ex- pressed by their ordinary formulae. It would have been interesting to have included sulphur in our list, but unfortunately the viscosity of liquid sulphur at tempei-atures but little higher than its melting poiut precludes observation by the method of registering ascent in a capillary tu be.It may be well here to recapitulate briefly that the variation of surface energy with temperature is, for non-dissociating liquids, a rectilinear function of the temperature, and that the value of the differential coefficients is, on the average, 2.121. Hence, for a normal or monomolecnlar liquid, the numerical value of z, or, as usually dE Et - ELI t t determined, 7 = 2.121, approximately. E represents mole- cular surface energy, or MU)^, where 7 is surface tension; 31, gaseous molecular weight ; v, specific volume ; and (Mc)~, molecular surface. If the value of M has been wrongly chosen, a coefficient differing from, and less than, 2-121 is obtained. In calculating the true value of M, it has been assumed that the principal cause of variation from * The experiments on phenol were made in conjunction with Dr.J. Shields.168 ASTON AND RAMSAT: THE MOLECCZAR FORMULE this general mean is association, and that the association (z) varies only slowly with temperature. Lek M, be the gaseous molecular weight. and :cN, the actual molecular weight of the liquid uuder es- periment,* then, Kt d { yzs(~otq)a} = 2.121, o r daf dt and &Co + r@,,z.)I. - = 2.121. d P d t It is assumed that xf = H/K,, and r(Mo.c)3. - is neglected as too small to affect the resnlt considerably. I n words, it is assumed that the alteration of association with temperature is negligible. It may be seen, by reference to the memoir on molecular surface energy (Phil. Truns., 184, A, 6551, that acetic acid, and methyl and ethyl alcohoIs do not exhibit a very rapid variation of association with temperature, and we therefore regard our assumption as justified for the limirs of temperature within which we determine tbe coeflicients of variation of molecnlar surface energy.In accordance with what has been said, it is easy to calculate the molecular weight of a liquid by the equation where the symbols have the meanings previously attacbed to them. 1c 1. Phenol.-The sample employed was colourless, and had a con- stant boiling point. We have taken as correct the densities as determined by Kopp (Annulen, 95, 312). It was easy to keep the substance liquid in the lower narrow tube, which held the lower part or stem of the capillary tube (see illnstratioa, Zoc.cit., p. '1094), by occasionally warming it with a Bunsen burner. Phenol (r = 0-01843 cm.). We now proceed to give the resuits of experiments. t. M. __I_- ----- 1-736 1-35 1269 ---- ------- 1.899 1'18 110.9 131 7 3-19 09670 27-90 1 589-9 184-0 2-70 09164 22-39 1 4W.6 I 1 9 Where the expression " actual molecular weight " or " mean molecular weight"OF LIQUIDS AND TEEIR MOLECULAR SURFACE ENERGY. 169 t. lo" 6 46.0 The molecular weight of gaseous phenol is 94 ; stud i t is seen that between 131.7" and 184", the value is rapidly approaching that number. In this it agrees with the alcohols ; and as it has been pre- viously noticed that the higher the molecular weight of acids and alcohols, the less the tendency towards association, it was to be ex- pected that a substance of such high molecular weight as phenol would show small tendency to form molecules of great complexity.Experiment has shown the correctness of this expectation. 2. Bromine.-The difficulty in determining accurately the capillary rise of bromine is that its opacity prevents accurate reading of the lerel of the lower meniscus. An approximate result, however, can be found by reading the upper edge of the meniscus; and if, as seems prabable, the height of this meniscus is not greatly altered by rise of temperature from 10" to 80", the results may be accepted as approximating to truth. Bromine (1' = 0.01046 cm.). h. -- rum. 2.4YO 2.230 -_ 78.1 1 1-972 P* 3 -152 3 031 2.917 I-- dynes 1 ergs 40.27 1 552'08 34-68 1 487-98 -- 29.51 1 426.09 1 M.202 -77 184 59 The densities at these temperatures were calculated from Thorpe's determinations (Trans., 1880, 37, 173). If it be supposed that the capillary rise should have been st milli- metre more in each case, the mean molecular weight is altered only to 201*SY, and 183.15 instead of 160, that of Br,. Hence a constant small error in reading would have little influence on the results. Dr. E. P. Perman showed in 1890 (Proc. Boy. Soc., 48, 49), that even at pressures near saturation, and at 15", the vapour density of bromine is normal ; and Paternb and Nasini, in 1888, found that in aqueous and in acetic acid solutions the molecular weight corresponded with Br2. It would appear that even in the liquid state most of the mole- cules possess that formula ; but some sign of association with fall of temperature appeara to take place.3. Nitric acid.-The sample of acid was prepared from nitric and sxphnric acid ; it was then mixed with phosphoric anhydride, and is used, it is not to be understood that the weight in question is that of a definite molecule, but is a mean result due to the fact that the liquid consists of moleculea of different complexity. For example, a molecule of H20 plus a molecule of H801 would have a mean molecular weight of 45.170 ASTON AND BAMSAY : THE MOLECULAR FORMULIE t. 11"-6 4.6 -2 - distilled over a water bath. A bulb was. filled, and when diluted and titrated, it proved to contain 99.8 per cent. of acid. It had a very pale yellow colour, and fumed strongly on exposure to the air.The tube was filled without warming the acid by exhausting a large globe provided with a stopcock, and attaching the neck of t h e latter with indin-rubber tubing to the drawn-out end of the experi- mental tube containing somewhat more than the requisite amount of acid ; on opening the stopcock, the acid evaporated under reduced pressure, and all air was expelled from the experimental tube. The tube was then sealed. The acid turned much darker on heating to 78", and it appeared to contain some nitric peroxide, but, on cooling, it regained its original pale yellow colour. h. -- 4-613 4,182 -- ATz'tric acid (r = 0.01192 cm.). 78-2 1 3.806 x . x x M. 1.681 105.9 --- 1.864 1 117'4 The densities were determined in a pyknometer at 11.6" and at 46-2". It was found impossible to prevent the acid from boiling at 'i8*2", although its boiling point is given as 86".The densitmy at 78.2" was, therefore, estimated by extrapolation. The results show that nitric acid in the liquid state has a molecular weight greater than that expressed by the formula HN03, and probably cousists of a large proportion of molecules of H,N206, mixed with simple mole- cules. 4. Sulphuric acid.-Acid of composition corresponding to the formula H,SO, is unstable, and dissociates on warming into sulph- uric anhydride and oil of vitriol, approximating to the composition 12H28Oc,H20. It was this acid which was employed for the experi- ments, andanalysis showed it to correspond exactly to the above formula. Its expansion was determined with the pyknometer at the tempera- tures at which its ascent in the capillary tube was measured.In calculating its molecular weight, the formula weight of the substance used has been assumed to be (H,SO,,&H,O) = 99.5 ; but, indeed, this is nncecessary, as the equation on p. 168 permits the determina- tion of molecular weight, M, without any assumption. The acid wets the tube easily ; it was sealed after warming in a Sprengel vacuum, so as to expel all dissolved gas. It was not viscous, but ascended easily,OF LIQUIDS AND TBETR MOLECULhR SURFACE ENERGY. 171 Kfromcurve. --- and gave concordant readings. table. The results are given in the annexed Sdpphuric mid (r = 0,01843 cm.). x x M. I 1 } 0.209 I 32.3 x 99.5 182.5 3.275 184.6 3.244 237 -7 I 3 -170 281-0 3.045 ---- I I 1'7342 51.35 763% ---------- 1'6874 49-40 749.7 I -6341 46 *84 724 -9 1-5912 43.80 690-1 - - ~ - - - - - - - ----- J 0.297 0 -599 1 -072 In mapping these results, it is obvious that the first four ralnes of y(Mo)j lie on a straight line ; the mean value of R read from the line is 0.209, and the molecular weight appears to correspond to (H,SO,),.Of course the water preeent may account to some extent for this extraordinary result, but we are disposed to regard it as true; for what is termed the boiling point of sulphuric acid, lying near 360", is in reality its temperature of dissociation under atmo- spheric pressure ; its true boiling point must lie much higher. Moreover, the existence OE complex snlphates such as NaK5(SO4>, implies a molecular weight corresponding to (H,SO,),.It is also worthy of remark that sulphuric acid possesses no appreciable vapour pressure at the ordinary tempeirtture ; for Johnson fouud (Chem. News, 68,211) that no sign of a sulphate could be detected in caustic soda exposed in a vacuum desiccator, containing sulphuric acid, even after R month. The liquid alloy of sodium and potassium also failed to absorb any vaponr from sulphuric acid after exposure for the same length of time. Now, under similar circumstances, a piece of gold leaf exposed to mercury vapour would have been heavily amal- gamated. It would be worth while making similar experiments to detect the possible volatility of such metals as cadmium and zinc. The small decrease of capillary rise with temperature and the corresponding small decrease of molecular surface energy with tem- perature are highly abnormal, and there appears no escape from the condusion that the molecular weight of liquid sulphuric acid is ex- ceedingly high.19'1 x 99.5 6.7 x 99.5 2 -8 x 99.5 - ------172 ASTON AND RAMSAY: THE MOLECULAR FORMULZ Above 130°, however, the molecular weight shows rapid diminn- tion. This may be due to one or to both of two causes, to the disso- ciation of such compound molecules, or to the partial dissociation of the compound into sulphuric anhydride and water. The method of experiment does not allow of a decision; but it is noticeable that faint fumes begin to be evolved from sulphnric acid at about 130", which would imply the latter conclusion, whether the former conclu- sion is true or not.We incline to ascribe the fall of molecular weight to both causes acting in concert. To calculate the moleculay weight directly from the individual observations is in this case not satisfactory, for the differences in h and consequent,ly in -1 are so small that errors of experiment tell very distinctly. But taking the whole range from 10.2" to 132-5", and calculating the molecular weight by the equation the number 3620 is obtained, which is not far removed from 99.5 X 32. But the other method is better adapted to eliminate error in this instance where several observations have been made. 5. Phosphorus.-This substance gave a great, deal of trouble, chiefly in discovering how to fill the tube. The phosphorus was first dis- tilled in a current of carbon dioxide and received i n a tube similar in form to the experimental tube, but having its narrow portion bent at an acute angle, and sealed to the experimental tube; a constricted portion between the two tubes was plugged tightly with glass wool, yet not so tightly as to prevent a stream of dry carbon dioxide from passing. After a sufficient quantity had distilled over, it was melted, and passed th~ough the glass wool into the experimental tube as a limpid liquid with a very pale yellowish tinge.The phosphorus was then melted away from the constriction, which was sealed. The Spreugel pump having been filled with dry carbon dioxide, the upper portion of the experimental tube was constricted, and secured to the Sprengel pump, and a vacuum was made ; the phosphorus was then melted, when a number of bubbles appeared, and were removed by pumping.It was at first thought advisable to boil the phosphortis in the vacuum, but this was found to produce some red phosphorus, and in later experiments the temperature was not raised so high as to cause ebullition. The tube was then sealed at the constriction, and jacketed with the usual vaponrs. The temperature of boiling carbon bisulphide is to3 little removed from the melting point Gf phosphorus to give gDod results ; hence higher temperatures were chosen. Liquid phosphorus does not satisfactorily wet g'lass, and to this also many of our difficulties are to be ascribed. It was only by sinkingOF LIQUIDS AND THEIR MOLECULAR SURFACE ENERGY. 173 t. - 780.3 132-1 the capillary tube by means of the magnet and raising it quickly that trustworthy readings were obtained ; after standing for a little, the cohesion between the phosphorus and the ~1;dss appeared to diminish. We fettr that it cannot be said that the angle of contact between phosphorus and glass is zero, as in the case of the other liquids in- vestigated ; but i t wonld be sufficient for our purpose if the angle is not materially altered within the limits of temperature employed.However this may be, the experiment yields a probable result, for the molecular weight found corresponds to the formula Po, as shown by the following t,able. h. P. Y. y (31 77) 1. I(. x. x x M. - ----- -- - 3-00 1.714 43-09 748 -2 2-55 1.664 3 5 5 6 629 -6 ------- 2.205 0.94 117.0 The molecular weight was here assumed to be 124, or 31 x 4, and we think that the difference is caused by experimental error. At all events, it may be said that there is no sign of association. In conclusion, it may be well to mention here that inasmuch as nitroet,hane was found to associate, an experiment was made with chloropicrin, C(NO,)Cl,, which gave normal results. The liquid is monomolecular. Our results shorn that phenol, like t,he alcohols? forms complex molecules in the liquid statre, which dissociate on rise of temperature ; that bromine, too, is somewhat associated, and aiso gives simpler molecular groupings of Br2 on rise of temperature ; that nitric acid consists largely of molecular groups of the magnitude (HNO,), ; that sulphuric acid is a very complex substance at ordinary tempera- ture, of probably not lower complexity tbati (H,SO,),, but tbat above 132" a rapid diminution of molecular weight is noticeable ; and that phosphorus in the liquid state has a molecular weight equal to that of its gas, P4. University College, London.