Page 10 - A TEXTBOOK OF ORGANIC CHEMISTRY

4 INTRODUCTION
proportion here is 7·81 to 6-30 = 1·239: 1, which corresponds
equally well with the numbers b:4 or Il:9. The formula
C,H, requires 93-75 p.c. carbon and 6·25 p.c. hydrogen, and
the formula C,H,, 93-62 p.. carbon and 6-38 p.c. hydrogen,
the deviations from the actual numbers found being in both
cases within the limits of experimental error. Therefore other
considerations must be taken into account here, in order to
decide between the two formule.
The formula derived from the results of analyses is termed
the Empirical Formula, and expresses the simplest numerical
relationship between the atoms of the elements present. 'The
actual molecular formula may be a multiple of this, and has to
be determined according to special principles.

Determination of Molecular Weight
1, By CHEMICAL METHODS.
Our chemical formulae (e.g. CH,O) express not merely a
percentage relation, but at the same time the smallest quantity
of the compound which is capable of existing as such, i.e. a
molecule of it. This molecule is ideally no longer divisible by
mechanical means, but only by chemical, and then into its con-
stituent atoms. If the formula CHO were the correct one for
acetic acid, then the amount of oxygen (or carbon) contained in
a molecule would be indivisible, and that of hydrogen divisible
only by 2. Since, however, it has been observed that one-
fourth of the total hydrogen in acetic acid is replaceable, e.g.
by a metal, with the formation of a salt, it is obvious that the
quantity of hydrogen in the molecule must be divisible by 4,
and so the formula must contain at least 4 atoms of hydrogen,
and must therefore be (HO,, or some multiple of it. This
is, in fact, the case. Acetate of silver contains 64·67 p.c. silver,
and therefore 35+33 p.c. of the acetate radical; or, to l atom of
silver = 108 parts by weight, there are D8 parts by weight of
the acid radical. This 59, together with 1 atom of hydrogen
=1, makes the molecular weight of acetic acid 60, = 2 x 30,
- 2x CH,O, = C,H,0,
This is a determination of molecular weight by chemical
means. Such determinations are carried out in the case of
acids generally by means of their silver salts; these are usually
normal salts, are easy to purify, are almost always free from
water of crystallization, and are readily analysed. It is,