The arrangement of symbols representing the chemical composition of substances. The symbols which are used are explained in the several sections dealing with Chemical Compounds,Chemical Interactions, and Elements, and the formulae employed are of various types. The ordinary formulae indicate the molecular composition without respect to any particular molecular arrangement or structure thus NaCl expresses the combination of 1 atom of sodium with 1 atom of chlorine in 1 molecule of salt. Again, the formula 2S04 expresses the combination of 2 of the groups NH4 with 1 of S04 in 1 molecule of ammonium sulphate, and so fortn. The empirical formula of a substance is ascertained from the results of its analysis, the percentages of elements found present being divided by their atomic weights. For example, hydrocyanic acid upon analysis is shown to contain as follows: and if these percentages are div ided by the atomic weights of the three elementsviz., II = x, C=i2, and N=i4it will be found that the quotients are practically identical, so that the component elements are combined in the proportions of one eachviz., HCN, which is the simplest formula of the substance. Or again, taking salt, it yields upon analysis 39316 per cent, sodium and 6o6?>4 per cent, chlorine, and the division of these numbers by the atomic weights 23 and 355 shows the two components to be contained in the elemental proportions so that the empirical formula is NaCl. Constitutional or Rational formulae, on the other hand, are used to express a sense of construction or structure, as when, for example, ammonium cyanate is resolved by heating into urea, the change may be expressed as follows: CNO = CO2 in order to show that, although empirically the composition of the two substances is the same, being concretely expressed by the formula CN2H40, the molecular arrangements or structures are different. Again, the empirical formula of ethyl alcohol is C2H#0, but when expressed constitutionally as C2H6, it is seen that the molecule has an alcoholic structure and that the group HO can be replaced or substituted by other groups. So again, acetic acid 2 may be expressed constitutionally in several ways, thus:

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Definition of  FORMULAE