Chemical formula determining from experimental data

By this point in your study of chemistry, you have seen hundreds of chemical formulas. Have you wondered where they come from or how we know the relative numbers of atoms of each element in a compound This section describes some of the ways chemists determine chemical formulas from experimental data. 

Determining a Chemical Formula from Experimental Data 114... 

So far the emphasis has been on solving the crystal structure from the knowledge of the unit cell and ionic content. The motivation for this work is to provide an automated procedure to help determine or solve the crystal structure of new compounds that are synthesised in a powder form. Of course the methods developed can generate other structural topologies and perhaps a new, yet to be synthesised, crystal structure. However, the task has been to solve a particular structure and so one might extract more information from the experimental data to aid the prediction process (e.g. use of symmetry elements). Thus, the number of unwanted possible (meta)stable structures, or polymorphs, that could be generated may be reduced. In this section, the emphasis is on finding all the important polymorphs for a particular chemical formula. 

The determination of the empirical formula of a compound can be made experimentally, by determining the percentage amounts of elements present in the substance using the methods of quantitative chemical analysis. At the same time the relative molecular mass of the compound has to be measured as well. From these data the empirical formula can be determined by a simple calculation. If, for some reason, it is impossible to determine the relative molecular mass the simplest (assumed) formula only can be calculated from the results of chemical analysis the true formula might contain multiples of the atoms given in the assumed formula. 

Careful analysis of Figure 4.20 indicates that six peaks in this card (43-0318) are clearly missing and three weak to medium intensity peaks have no match in the experimental pattern. The second record 49-1170, is almost a perfect match, but a hydrogen atom is missing in its chemical formula, as was determined later from neutron diffraction data. ... 

The set-up of elemental and enthalpy balances constitutes the basis for the consistency check of experimental data from biotechnological processes. For that purpose, the exact composition and physico-chemical characterization of all species involved in the process under study has to be determined including biomass. Another use is the design of the growth medium or the investigation of possible stoichiometric limitation by the medium supply. We chose to take into account all atoms bound in the main macromolecules whereas other atoms, mainly present as ions in biomass (K, Ca, Na. ..), are not incorporated in the elemental formula [28]. One C-mol of biomass is defined as C.H, O, N S, R... 

Using the freezing-point data is another experimental technique that can be used to obtain the chemical properties of a substance. In this example we were able to determine the molecular formula from the freezing-point depression and a known amount of substance dissolved in a solvent. Note that water was used as the solvent here however, other solvents can be used in freezing-point experiments. 

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