VAPOR-DENSITY :METHOD. The molecular weights of gases, or of substances that can be obtained in the state of vapor, can lie readily determined by ascertaining the density of the gas or vapor. The principle involved is as fol lows: The density (D)—i.e. the weight of a cer tain volume of gas—is obviously the product of the number (u) of molecules contained in that volume, and the weight (AI) of each molecule: D OE, whence D = n.
Now, according to Avogadro's rule, equal vol umes of all gases and vapors contain the same number of molecules, provided the temperature and pressure are the same; in other words, the number (n) contained in a given volume of gas does not vary with the nature of the latter; it is the saute for all gases. For any two different gases we shall therefore have D, D., Ai, Al, ' whence AI, D, = D2' i.e. the molecular weights of gases are propor tional to their densities. Evidently, AI,,= so that. if D, and AI, stand for the density and weight of a molecule of hydrogen (the lightest known gas), the weight of a molecule, say, of oxygen, may be found by ascertaining its density referred to hydrogen (i.e. the weight of any volume divided by the weight of the same volume of hydrogen at the same pressure and tempera ture) and multiplying that density by the weight of a molecule of hydrogen. Now, if in stead of multiplying the densities of gases by the true weight of a molecule of hydrogen, We mul tiply them by any arbitrary number, we obtain, instead of the true weights of the molecules of gases, a series of other numbers whose values depend on that of our arbitrary number for hydrogen. But, if we adhere to the same ar bitrary number, our numbers for the various gases will bear the ratios to one another as the true weights of their molecules. And since in studying substances and reactions we need know not the absolute, but the relative weights of molecules, we may assign to our standard gas, hydrogen, any arbitrary number whatever. For reasons explained in the article ATOMIC WEIGHTS, t he molecular weight of hydrogen is assumed to be 2.
Therefore, to determine the molecular weight of a volatile compound, all a idiemist has to do is to ascertain the weight of it given volume of vapor. to divide that weight by the weight of an equal volume of hydrogen gas at the saute pres sure and temperature, and to multiply the vapor density thus obtained by 2. Water vapor is found to be times as heavy as hydrogen gas; hence, its vapor density is said to be :1; multi plying by 2, the molecular weight of water is seen to be 18. The vapor density of chemical compounds is usually determined with the aid of one of the following apparatus: A. Dumas.'s A/prim/us consists of a light glass flask (of about 230 cubic centimeters capacity). whose neck is bent and drawn out into a long and narrow point. After carefully weighing the flask, a few grams of the substance to be ex amined are introduced into it; it is then im mersed in a bath whose temperature is constant and somewhat higher than the boiling-tempera ture of the substance in the flask.
The substance soon begins to boil. and its vapors drive all the air out of the flask. When the substance has com pletely evaporated and the flask contains noth ing but its vapor. the open end is carefully sealed off with the aid of a blowpipe. On cool ing, the flask is cleaned and again weighed. Fi nally, its end is broken Orr under water, which rushes into the flask owing to the low pres sure within; and when the latter is complete ly filled, it is weighed a third and last time, together with the end that has been broken off. From the three weighings, the weight and the volume of the vaporized substance in the ap paratus become known. Dividing this weight by that of an equal volume of hydrogen at the temperature of the bath and the barometric pres sure which the •determination has been carried out, we get the vapor-density, and from this, by multiplying by 2. the molecular Weight of the substance examined. Dumas's apparatus can be employed only when a considerable amount of substance is available.