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Valency : Determination Of Valency
The valency of an element can be determined with certainty only (i) from compounds containing a single atom of the element, (ii) if the molecular weight of the compound is known, and thence its molecular formula.
The molecular weight of a substance can be found from vapour density and also in solution. Hence the compound must be volatile, or must dissolve without decomposition in a solvent, in order that its molecular weight, and the valency of a constituent element, can be found. The presence of certain groupings of atoms in a compound may often be inferred from chemical reactions, and this may lead to the structural formula of the compound.
Sulphuric acid, H2SO4, for example, when treated with phosphorus pentachloride, forms the two compounds HClSO3 and SO2Cl2 in succession:
H2SO4 + PCl5 = HClSO3 + POCl3 + HCl, HClSO3 + PCl5 = SO2Cl2 + POCl3 + HCl.
In each of these reactions the hydroxyl radical, OH, is replaced in the acid by an atom of chlorine, so that the formula of sulphuric acid may be written SO2(OH)2, and that of the compound HClSO3 as SO2(OH)Cl. The sulphuryl radical SO2 (which is not the same as the sulphur dioxide molecule) must, therefore, be bivalent, since OH and Cl are univalent. If we assume that the sulphur atom has its maximum valency of six, as in SF6, the structures of sulphuric acid and of the two other com pounds may be represented by the formulae:
The following formulae have been found from direct measurements of vapour densities, in some cases (e.g., AgCl at 1735°) at very high temperatures:
In a few cases the valency has been confirmed by the vapour densities of volatile organo-metallic compounds: zinc methyl ZnII(CH3)2, lead tetraethyl PbIV(C2H5)4, aluminium acetylacetonate AlIII(C5H7O2)3, and tin triethyl Sn2IV(C2H5)6.
The valency of an element may be determined from the ratio of the atomic weight to the equivalent. According to Dulong and Petit's law, the product of the specific heat and atomic weight of a solid element is constant, and equal to 6-3. If the atomic weight is found in this way, and divided by the equivalent, determined by a particular method, the valency of the element is known. In some cases the equivalent may be determined by making use of Faraday's law of electrolysis.
The equivalent of zinc, determined by the amount of hydrogen evolved by the action of zinc on dilute acid, is 32.5. The specific heat of zinc is 0.0955, hence the atomic weight is approximately 6.3/0.0955=65. But 32.5x2=65, hence the valency of zinc in the chloride and sulphate is 2, and the formulae of these compounds are ZnCl2 and ZnSO4.
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