Page 11 - A TEXTBOOK OF ORGANIC CHEMISTRY

MOLECULAR WEIGHT DETERMINATION 6
however, absolutely necessary to know whether the acid is mono-
or polybasic. In the case of a di-, tri-, &e., basic acid, the above
calculation must be made with reference to 2, 8, &e., atoms
of silver, whereas acetic acid--being monobasic-contains
only one replaceable atom of hydrogen, which is therefore
exchanged for one atom of silver. Consequently, its formula
cannot be a multiple of C,1H,O,
In the determination of the molecular weights of Bases, their
platinichlorides are similarly made use of, these being almost
always constituted on the type of ammonium platinichloride:
(NH),H,Pt&l: i.e. they contain two molecules of a mono-
acid base such as ammonia combined with one atom of
platinum,
To determine the molecular weights of Neutral Compounds,
derivatives must be prepared and examined for the proportion
of the total hydrogen which is replaceable, e.g. by chlorine.
For example, by the action of chlorine upon naphthalene,
there is first formed the substance monochloronaphthalene,
which contains 73-8 per cent C, 4-3 per cent H, and 21·9 per
cent CI, these numbers giving the formula CH,CI. In the
same way benzene yields the compound C,H,Cl. In both
these cases the halogen acts by replacing hydrogen, and at
least one atom of the latter in the molecule must be replaced,
since fractions of an atom are necessarily out of the question.
If, then, the compound obtained has the formula (~H,CI, it
follows that ] of the H present has been replaced by Cl, and
there must consequently be 8, 8 x 2, or 8 X 3, &c., atoms of
hydrogen in the compound, and likewise I0 atoms, or some
multiple of 10, of carbon. But a multiple of 8 or 10 may be
rejected, since no compounds have been observed which would
indicate the replacement of e of the total hydrogen. 'This
leads to the formula (H, for naphthalene, the other possible
formula got by analysis, viz. (H, (see p. 4), being now
untenable. In a similar way the formula of benzene is found
to be 0,H,
2. BY PHYSICAL METHODS.
The commoner methods used are:
I. Vapour density method.
2. Cryoscopic method.
2. Ebullioscopic method.
4. Vapour pressure method (for details et. S. J., Chap. VIII).

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