Composition of Chemical Compounds

A chemical formula conveys considerable quantitative information about a compound and its constituent elements. We have already learned how to determine the molar mass of a compound, and, in this section, we consider some other types of calculations bas on the chemical formula. 

The colorless, volatile liquid halothane has been used as a fire extinguisher and also as an inhalation anesthetic. Both its empirical and molecular formulas are C2HBrClF3, its molecular mass is 197.38 u, and its molar mass is 197.38 g/mol, as calculated below  

The molecular formula of C2HBrClF3 tells us that per mole of halothane there are two moles of C atoms, one mole each of H, Br, and Cl atoms, and three moles of F atoms. This factual statement can be turned into conversion factors 

In Example 3-3, we use another conversion factor derived from the formula for halothane. This factor is shown in blue in the setup, which includes other familiar factors to make the conversion pathway  

EXAMPLE 3-3 Using Relationships Derived from a Chemical Formula 

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The atomic theory provides a ready explanation for the definite composition of chemical compounds. It says that compounds are composed of atoms, and every sample of a given compound must contain the same relative number of atoms of each of its elements. Since the atoms of each element have a characteristic weight, the weight composition of a compound is always the same. Thus, the definite composition of compounds provides experimental support for the atomic theory. 

During the latter part of the nineteenth century and the early years of the twentieth century, there was considerable controversy over the composition of chemical compounds—were compounds strictly stoichiometric, with an immutable composition, or could the composition vary. Indeed, at the turn of the twentieth century, even the existence of atoms was a subject of debate. The principal techniques involved at this epoch were accurate quantitative chemical analysis and metallo-graphic studies of phase equilibria. The advent of X-ray diffraction studies effectively resolved the problem, and the experimental evidence for composition ranges of many solids became incontestable. 

Impulse for the valuation of the chamomile collection has become if the identification of four chemical types of this plant species different by the qualitative -quantitative composition of chemical compounds in the essential oil was carried out by Schilcher in 1987 (Table 7.6). This veiy important fact was referred to chamomile biodiversity. This biodiversity was created during long time process (evolution) inregardtoinfluenceofeco-physiological conditions (biotic-andabiotic-factors) on the concrete place of chamomile population growth. 

This empirical tradition was challenged toward the end of the century by the theoretical work of Claude Louis Berthollet. Like many chemists of the eighteenth century, he saw affinity as properly investigated in the context of Newtonian forces between the chemical particles. Berthollet believed that chemical affinity was determined not only by the intrinsic chemical forces, but also by the chemical masses of the reacting bodies, a view that would also lead to variable composition of chemical compounds. 

During the latter part of the nineteenth century and the early years of the twentieth century, there was considerable controversy over the composition of chemical compounds. It was clear that some solids, especially metallic alloys see... 

Nowadays, spectrophotometry is regarded as an instrumental technique, based on the measurement of the absorption of electromagnetic radiation in the ultraviolet (UV, 200-380 nm), visible (VIS, 380-780 nm), and near infrared region. Inorganic analysis uses UV-VIS spectrophotometry. The UV region is used mostly in the analysis of organic compounds. Irrespective of their usefulness in quantitative analysis, spectrophotometric methods have also been utilized in fundamental studies. They are applied, for example, in the determination of the composition of chemical compounds, dissociation constants of acids and bases, or stability constants of complex compounds. 

Chemists prefer systematic names that precisely identify the chemical composition of chemical compounds. The system for inorganic nomenclature was devised by the International Union of Pure and Applied Chemistry (lUPAC), which was founded in 1921. The lUPAC meets regularly and constantly reviews and updates the system. 

An analysis of the core probes from the sediment of the canal indicated high concentrations of total hydrocarbons (70-80 cm - 19.1 g kg" ) and PAHs (70-80 cm - 157.5 mg kg" ) in all layers. There was no noted regularity in changes in the concentrations of total hydrocarbons and PAHs with increase in the depth of the sediment. The interdependence between concentrations of total hydrocarbons and PAHs was clearly expressed. It should be mentioned that polychlorinated biphenyls were not found. On the basis of mass spectrum analyses of the undisturbed sediment and the composition of chemical compounds, it can be concluded that the dominant pollution is of petroleum origin. 

When Proust began his work the constancy of composition of chemical compounds was generally accepted and it formed the basis of the quantitative analytical methods of Bergman (p. 187), Wenzel (p. 671), Kirwan (p. 664) and Richter (p. 681). Lavoisier, although he generally assumed that different compounds formed from two elements were definite in composition, and recognised the existence of two oxides of copper (p. 461), said ... 

Bergman, Wenzel, and Kirwan had published experiments on the composition of chemical compounds, and it cannot be said that Richter was working in a field which was neglected, or in which chemists as a whole took little interest. He proved that the analysis of a few neutral salts could furnish by calculation the compositions of many others, and so control the accuracy of analyses. The compositions of neutral salts could be calculated from numbers characteristic of the acids and bases. He proved the constancy of the ratios of the quantities of different acids or bases neutralising the same quantity of any one base or acid, and the constancy of the quantity of oxygen in the quantities of various metal oxides neutralising the same amount of an acid and by inference that metals combine with fixed amounts of oxygen, which in the case of metals such as mercury, which form two series of salts, are in a fixed ratio. He failed to notice the simple multiple proportions in the last case, not by reason of... 

Dalton s and Avogadro s theories became cornerstones of modem science. They also gave simple explanations of the constant compositions of chemical compounds and to Proust s law of definite proportions. 

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