Page:The New International Encyclopædia 1st ed. v. 13.djvu/762

* MOLECULES. 684 MOLECULES. lected in a gradualcd tiilio, and thus the voluinc of the vai)oiizc'd sub^itancc t>cconies known. Di- viding the «eiyht of the substance taken by the weiglit of an cqiuil volume of hydrogen, at the temperature of the vaporized substance in the apparatus and at the Ijarometric pressure under which the determination has been carried out, we VK'TOR .MEYER 8 APPARATUS. get the vapor-density, and from this, by multiply- ing by 2, the molecular weight of the substance. Victor Meyer's apparatus yields jirccise results and requires but a very small quantity of the substance experimented upon. The latter cir- cumstance forms an important advantage : for the preparation of large (piantities of a substance often involves fireat loss of time and is some- times very dilliciilt. Method of ])ktf;rmi nation of Molf.cul.u WeICHT.S nv DiUKCT or IxniRECT ilE..SVRi;MENT OF THE Osmotic I'hkssire of Solitions. In the gaseous state a small nuiss of substance occupies a large volume of space, and the properties pecu- liar to the gaseous state arc due to the fact that the molecules of a gas are separated from one an- other by considerable distances. But a little sugar dissolved in much water is also distributcil throughout a large volume; (he molecules of su- gar are likewise separated from one another by considerable distances, and this is why the ])rop- orties of sugar in dilute solution are very much like what they would be if the sugar existed in the gaseous state. Thus, within the volume of a solution, sugarfas well ns any other substance) has been proved to exert a pressure equal to that which it would exert were it vaporized and in- closed within a vessel whose volume is eqiial to that of the solution. The pressure of a substance in solution is called osiiinlir prcusiirr. When equal weights of dilTercnt gases are confined within equal volumes, the pressures are inversely proportional to the iiiulciular weights of the gases. Thus, if eiiual quuiititii's by weight of oxygen and hydrogen were inclosed within vessels, say, of one liter capacity, the pressure in the vessel containing hydrogen would be IG times as great as the pressure in the vessel containing oxygen ; while, on the contrary, the molecular weight of hydrogen (2) is .^'j the molecular weight of oxygen ( 32 ). The reason is obviously this: the lighter the molecules, the greater must be their number, to constitute a given weight of gas; but the greater the number of molecules, the greater the iiressure of the gas; hence, the lighter the molecules, the greater the pressure of a given mass of gas. .Similarly, in the ca.sc of equal weights of difTerent substances in solution, the osmotic pressures are inversely proportional to the molecular weights. The relative molecular weights of soluble substances may therefore be found by preparing solutions having equal vol- umes and iiolding equal weights of the dissolved substances, then measuring the osmotic pressures of the solutions. Sec Soi.iTlo.v. But as the direct measurement of osmotic pres- sure is exceedingly dillicult, indirect methods of measurement are usually employed by the in- vestigator. When a substance enters some sol- vent, say water, it changes the characteristic freezing and boiling temperatures of the solvent: the freezing-point is lowered, the boiling-point is raised. Xow. the depression of the freezing-point and the elevation of the boiling-point have been shown to be proportional to the osmotic pres- sure of the solution. Therefore, instead of carry- ing out a direct determination of osmotic pres- sure, the chemist measures the ehanj.'e of freezing or boiling temperature caused by dissolving a given weight of substance, and calculates the molecular weight of the latter on the principle that the change caused by a substance is in- versely proportional to its molecular weight. The laboratory methods commonly employed in de- termining the depression of freeziiig-]ioint and the rise of boiling-point are descrilied in the articles Freezing-Point and Boii.inc-Point. The direct and indirect osmotic- pressure meth- ods just referred to ;ue of the greatest practical importance to (he chemist. For while the number of substances that can be vaporized (:ind hence whose molecular weights can be determined by (he vapor-density method) is limited, practically all substances can be obtained in solution. C11E.MICAL Metiioos. It is often possible to determine the molecular weight of a substance by purely chemical methods, i.e. by studying its formation and chemical (ransfonnations. Con- sider, for example, aeefic acid, . alysis leadi us to assign to it one of the f(dlowing formulas: CH..O, C.H.O., C'-,II,A, C,H,0„ etc., corresponding to "the molecular weights, ."iO, CO, !10, 120, etc. (Sec CiiEMl.sTRY.) The fact that acetic acid is readily formed from ordinary alcohol, whose molecule (C;H.,0) cannot possibly contain less than two carbon atoms (because for every two carbon atoms it contains a single atom of oxy- gen) — this fact renders it ]irobabIe that the molecule of acetic acid, too, contains two carbon aloins. In odier words, it becomes jirobable that f';ll,(1j is the formula, and hence (If) (he molecu- lar weight, of acetic acid. That ClI/) cannot be (he formula of acetic acid is shown by the fact that the molecule (CjHjOjAg) of silver ace-