Imagini ale paginilor
PDF
ePub

These being divided by the respective atomic weights, the quotients will express the ratio of the elementary atoms in the compound.

[blocks in formation]

If these are now reckoned to 1 atom of carbon they give—

[blocks in formation]

or as nearly as can be expected with unavoidable errors, to the empirical formula CH2O, from which the following percentage composition is calculated:

C = 40.00

H = 6.67
0 = 53.33

100.00

Determination of the Molecular Formula.

21. The chemical formula of a body must be something more than the mere expression of its percentage composition, it should in addition express the atomic composition of the smallest, relatively, existing quantity of the body, its molecule. It is evident that the latter can be any whole multiple of the empirical formula; the acetic acid molecule may probably be C2H4O2, C3H6O3, &c.

In order, therefore, to find the true chemical formula of a body, in addition to the percentage composition there must also be determined the relative weight of its molecule, either from certain physical qualities, especially the vapour density (see further on), or from the products derived from it by chemical changes.

[ocr errors]

Of these latter derivatives' the most important for acids and bases are their salts; for indifferent bodies, especially those that contain only carbon and hydrogen, their haloid substitution products, i.e. derivatives in which the hydrogen is replaced by a halogen.

22. When acetic acid, for instance, is converted into its silver salt by boiling with argentic oxide, and the salt then submitted to analysis, it gives the following percentage composition :

[blocks in formation]
[blocks in formation]

and requires for acetic acid a molecular formula at least as large as C2HO2, i.e. double the empirical formula previously determined.

Several other substances have the same percentage composition as acetic acid, although with dissimilar properties; such are dried grape sugar, lactic acid, &c. The analysis of derivatives of these bodies, however, leads to completely different molecular formula.

The silver salt of lactic acid, e.g., gives this latter the formula C3H6O3, as it contains three atoms each of carbon and oxygen and five atoms of hydrogen to one atom of silver.

Found per Cent. Atomic Quotient Found

[merged small][ocr errors][merged small][merged small][ocr errors]

To 1 atom Ag

Calculated.

[ocr errors][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small][ocr errors][ocr errors][merged small][merged small][merged small]
[ocr errors]
[merged small][ocr errors][merged small]

23. In order to determine the molecular formula of an organic base it is analysed both in the free state and in form of the salts which it yields with acids-most simply with hydrochloric acid. Bearing in mind that organic bases, or alkaloids, resemble ammonia in their chemical behaviour, i.e. unite with acids without separation of water, it is easy from the composition of the salt to deduce that of

the base.

As an example the formula of the hydrochloride compound of creatinine may be calculated. On elementary analysis it gave in per

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors]

On calculating this to 35.5 parts, or one atom, of chlorine there was

found

[merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small]

8

The formula of creatinine hydrochloride is therefore C,H,N3OCI. On subtracting HCl from this we get for creatinine itself C,H,N ̧0, with which formula the results of the analysis of free creatinine are in agreement.

[merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

24. As the so-called indifferent bodies-i.e. such as are neither of acid nor basic nature-do not enter into combination, it is necessary, in determining their molecular weight, to have recourse either to splitting up-i.e. decomposition into several other compounds of known molecular formula, whose relative quantities are then determined—or to substitution.

The analysis of acetic ether, e.g., gives as its empirical formula C2HO; by treatment with potash solution, however, it splits up into equal molecules of ethyl alcohol, C2HO, and potassic acetate, C2H KO2. It is therefore seen that the above expression must be doubled in order to get four carbon atoms. The decomposition is then represented by the equation:

C1H ̧O2+KOH=C2H ̧O+C2H3K02.

8

25. The simplest formula derivable from the analysis of benzene is CH. A crystalline compound of benzene and chlorine appears to confirm this, as its investigation leads to the formula CHCl. When heated with alcoholic potash, however, it is converted into an cil, which to one chlorine atom contains two carbon and one hydrogen

atoms.

From this the benzene formula would be C2H2, the chlorine compound C2H2Cl2, the decomposition product C,HCl. By the investigation of other products of the action of chlorine upon benzene, C2H2 proves not to be its formula. One of these bodies contains three carbon atoms to one chlorine atom = C3H2Cl. The composition of another corresponds to C3HC12. Already these derivatives with both two and three carbon atoms require the presence of C in the molecule, and apart from that other substitution products exist that cannot be otherwise formulated.

From the empirical formula of the chlorine derivatives the following series of molecular formulæ of benzene derivatives is obtained :

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][ocr errors][ocr errors][ocr errors][merged small][merged small][ocr errors][ocr errors][ocr errors][merged small][merged small][merged small][ocr errors][ocr errors][ocr errors][merged small][ocr errors][merged small][ocr errors][ocr errors][ocr errors][merged small][merged small][merged small][merged small][ocr errors][ocr errors][ocr errors][ocr errors][merged small]

Derivation of the Molecular Weight from the Vapour Density.

26. The fact that in the union of gases the volumes stand in very simple relation to each other and to the volume of their gaseous compound, led at the beginning of this century to the view that equal volumes of different gases and vapours contain, under like conditions of temperature and pressure, an equal number of molecules. This has since been amply confirmed, and now forms one of the most important fundamental laws of physical chemistry.

The weights of equal volumes of different gases, under like temperature and pressure (the gas and vapour densities), express directly the relative weights of the molecules. The molecular weight of any gaseous or volatile (without decomposition) body is found by multiplying the experimentally determined density on the air scale (i.e. air 1) by 28-92-i.e. the molecular weight is 28-92 times as great as the density of the gas or vapour on the air scale.

=

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small]

Relatively small errors in the determination of the vapour density would lead to not inconsiderable deviations in the molecular weight obtained by this method; but as the molecular weight must be a whole multiple of the empirical formula, the results are quite sufficiently near to leave no doubt as to which multiple is the correct one.

Determination of Vapour Density.

27. The density of a gas or vapour is the quotient of a given volume thereof divided by the weight of an equal volume of atmospheric air at the same temperature and pressure. As a cubic centi

metre of atmospheric air weighs 0012932 grm. at 0° and 760 mm. pressure, according to the laws of Marriotte and Gay-Lussac, the weight (in grammes) P of any given volume v (in cc.) at any given temperature t and pressure b can be calculated by the formula

[merged small][merged small][ocr errors][merged small][merged small]

There is only required, therefore, in the estimation of a vapour density, the weight p' and the corresponding volume v' at the temperature t and pressure b of the gaseous or vapourisable body. There are two different methods employed for this purpose: either the amount required to fill a flask of known capacity at a known temperature and pressure is weighed, or the volume occupied by a known weight of the vapour is measured.

28. In order to estimate the density according to the first principle, a glass balloon of about 300-500 cc. capacity is employed; the neck is drawn out to a long cavillary tube, and bent at an obtuse angle.

C

The flask is dried by repeated exhaustion and admission of dried air, and then weighed full of dry air, the temperature t and pressure b being noted. About 5-10 grm. of the liquid under examination is introduced into the slightly warmed balloon, which is then placed in

FIG. 13.

an oil bath, heated to a few degrees above the boiling point of the substance, as shown in fig. 13. The thermometer must be so placed in the oil that its bulb is as high as the middle of the balloon and as near as possible to it. The air is expelled by the vapour formed, the excess of the latter also escaping. When this ceases the temperature is raised about 20°-30°; the portion of the capillary tube projecting above the oil being also heated, to volatilise any liquid condensed there; the point fused in the blowpipe flame, the temperature t, and the pressure b being noted at the same time. The balloon is now

removed from the oil bath, carefully cleaned, and when cold weighed. The next point is to ascertain the capacity of the balloon and the volume of any air left in it. The point is, therefore, broken off under mercury, whereupon the balloon fills with the metal. If a gas bubble of sufficient size to affect the result is visible, it is transferred to a graduated tube placed over mercury, and its volume and weight determined, calculated also to the temperature of the bath and pressure at the time of fusing, and both numbers used for the correction of the weight and volume of the vapour.

The balloon, after completely filling with mercury, is then emptied into a graduated vessel, in order to determine its capacity at the ordinary temperature. The following data for the calculation of the vapour density are now obtained :

P weight in grammes of the balloon filled with air at the temperature t and the pressure b.

P' weight in grammes of the balloon filled with vapour at the temperature t' and the pressure b'.

v capacity of the balloon at the temperature t.

From these the weight p of the atmospheric air in the balloon at t and 6 mm. pressure is thus calculated.

[graphic]
[merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

(PP).

The glass expands on heating; therefore the capacity v' of the

« ÎnapoiContinuă »