A cubic foot of oxygen weighs 16 times as much as a cubic foot of hydrogen.
It has been stated that equal volumes of gases contain the same number of molecules. Therefore, a cubic foot of oxygen must contain the same number of atoms as a cubic foot of hydrogen. Since the former weighs 16 times as much as the latter, it follows that an atom of oxygen weighs 16 times as much as an atom of hydrogen. Similarly, an atom of chlorine weighs 35.5 times as much as an atom of hydrogen. This ratio between the weight of an atom of any element and the weight of an atom of hydrogen is called the atomic weight of the, element. The atomic weight of any element may be found by dividing the weight of a given volume, say 1 cubic foot, of the element, when in a gaseous state, by the weight of 1 cubic foot of hydrogen. The atomic weight, there- fore, bears a direct relation to specific gravity.
8. The atomic weights of the elements most often found in fuels are given in Table II.
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TABLE II Elements Atomic Weights Hydrogen, H Oxygen, O Nitrogen, N Carbon, C Sulphur, S 1 16 14 12 32
With the aid of these atomic weights, the composition of any substance by weight can be found when its formula is known. .Take, for example, water, H2O, which has two atoms of hydrogen to one of oxygen, and multiply the number of atoms of each by the atomic weight of the element; the results will be the parts by weight of the elements. Thus,
2 atoms of H × atomic weight, 1 = 2 parts of H 1 atom of O × atomic weight, 16 = 16 parts of O 18 parts of H2O
Then, the water is composed of 2⁄18 = 11.11 per cent, of hydrogen, and 16⁄18 = 88.89 per cent, of oxygen.
