Compounds counting molecules

The effect of the solute on the vapor pressure of a solution gives us a convenient way to count molecules and thus provides a means for experimentally determining molar masses. Suppose a certain mass of a compound is dissolved in a solvent, and the vapor pressure of the resulting solution is measured. Using Raoult s law, we can determine the number of moles of solute present. Since the mass of this number of moles is known, we can calculate the molar mass. 

If you missed 18, go to Compounds Moles and Counting Molecules, page 123. 

In addition, alternate virtual screening methods can incorporate activity-class-characteristic features. For example, by counting the number of activity-characteristic features in test compounds, active molecules were identified that possessed new combinations of activity-characteristic features that were not observed in reference compounds, resulting in chemotypes that were distinct from any of the reference compounds [13]. Similarly, virtual fragment linking [7] used features enriched... 

FIGURE 5.59 More of the naming protocol for bridged bicyclic compounds. This molecule is 3,3-dimethylbicyclo[3.2.1 ]octane. The red dots show the bridgehead atoms. These are not counted in sizing the bridges, but are counted in numbering the compound. 

When we prepare a compound industrially or even study a reaction in the laboratory, we deal with tremendous numbers of molecules or ions. Suppose you wish to prepare acetic acid, starting from 10.0 g of ethanol. This small sample (less than 3 tea-spoonsful) contains 1.31 X 10 molecules, a truly staggering number. Imagine a device that counts molecules at the rate of one million per second. It wonld take more than four billion years—nearly the age of the earth—for this device to count that many molecules Chemists have adopted the mole concept as a convenient way to deal with the enormons numbers of molecules or ions in the samples they work with. 

Clearly, the next step is the handling of a molecule as a real object with a spatial extension in 3D space. Quite often this is also a mandatory step, because in most cases the 3D structure of a molecule is closely related to a large variety of physical, chemical, and biological properties. In addition, the fundamental importance of an unambiguous definition of stereochemistry becomes obvious, if the 3D structure of a molecule needs to be derived from its chemical graph. The moleofles of stereoisomeric compounds differ in their spatial features and often exhibit quite different properties. Therefore, stereochemical information should always be taken into ac-count if chiral atom centers are present in a chemical structure. 

As the C B series of tetracarbaboranes is classified in the electron-counting formaUsm as nido, these molecules are expected to have open stmctures even though extra hydrogens are absent. Spectroscopic studies (130) have confirmed this expectation for 2,3,4,5-C4B2H3 [28323-17-3]. One isomer of (CH3)4C4BgHg has the open nonicosahedral stmcture shown in Figure 11 and another isomer, the 1,2,3,8-tetramethyl compound [54387-54-1], is apparently even more open (131). Other tetracarbaboranes include isomers of nido-Q]. and (132). 

The structure of cluster compounds of transition metals and the limits of applicability of the electron counting rules forpolyhedral molecules. Y. L. Slovokhotov and Y. T. Struchkov, Russ. Chem. Rev. (Engl. Transl), 1985, 54, 323 (150). 

Regardless of their possible metallic properties, metal-rich Zintl system or phases are defined here as cation-rich compounds exhibiting anionic moieties of metal or metalloid elements whose structures can be generally understood by applying the classical or modern electron counting rules for molecules. 

To determine a chemical formula, we would like to count the atoms of each element in one molecule of the compound. Atoms are too small to count, but we might hope to measure the number of moles of each element present in one mole of the compound. Unfortunately, there is no direct experimental method for measuring moles. Instead, laboratory experiments give the masses of the various elements contained in some total mass of the compound. 

One of the first decisions to be made when designing an experiment is the method of detection to be used with a particular solute. If radiolabeled material is available, a simple method of analysis is to count the radiolabel appearing in the receiver compartment as a function of time. While convenient, this can be a dangerous practice. Depending upon the type of radioisotope, its position in the molecule, and its specific activity, radiolabeled compounds can be subject to a variety of chemical and solution-catalyzed degradation pathways. If the stock solution contains a significant amount of radioactive impurities or generates them as a result of solution instability, then the possibility for preferential transport of... 

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