Stoichiometry and Solution Chemistry

Chemical Formulas Balancing Chemical Equations Mole Ratios 

Volumes, and Molecules Limiting and Excess Reagents Percent Composition 

Stoichiometry is the branch of chemistry that deals with the amounts of products produced from certain amounts of reactants. Most of the chemistry discussed so far in this book has dealt with what is present (qualitative chemistry). The next step is to examine how much is present (quantitative chemistry). You may want to refer to Appendix 1, Mathematical Skills Review, in the back of this book. 

Chemical formulas not only tell which elements are present in a compound but also how much of each element is present. There are three different types of chemical formulas that need to be examined. The chart below shows the major differences between them. 

Symbols You might think that chemical symbols tell only which 

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A. Jayasankar, L.S. Reddy, S.J. Bethune, N. Rodriguez-Hornedo, Role of cocrystal and solution chemistry on the formation and stability of cocrystals with different stoichiometry, Cryst. Growth Des. 9 (2009) 889-897. 

Citrates. Iron citrate [2338-05-8] is a compound that contains citric acid and iron(II) and iron(III) in indefinite ratios. Iron(II) citrate [23383-11-1] and iron(III) citrate [28633-45-6] are also of indefinite stoichiometry, although iron(III) citrate which contains Fe and citric acid in a 1 1 ratio [3522-50-7] is known. These compounds dissolve slowly in water and are more readily soluble in hot water. The solution chemistry of these compounds is comphcated by formation of a number of monomeric and oligomeric species. All of the iron citrate compounds are used as supplements to soils and animal diets. 

Still lower pHs to give a pale-yellow solution. As a result of spectrophotometric studies there is general agreement that the predominant species in the initial colourless solution is the tetrahedral V04 ion and, in the final pale-yellow solution, the angular VOz ion. In the intervening orange to red solutions a complicated series of hydrolysis-polymerization reactions occur, which have direct counterparts in the chemistries of Mo and W and to a lesser extent Nb, Ta and Cr. The polymerized species involved are collectively known as isopolymetallates or isopolyanions. The determination of the equilibria involved in their formation, as well as their stoichiometries and structures, has been a confused and disputed area, some aspects of which are by no means settled even now. That this is so is perfectly understandable because ... 

Numerous investigators have attempted to control the precursor structure and related solution chemistry effects with varying degrees of success, to influence subsequent processing behavior, such as crystallization tempera-ture.40-42,78,109 110 Particular attention has been given to precursor characteristics such as structural similarity to the desired product and the chemical homogeneity of the precursor species. For multicomponent films, this latter factor is believed to influence the interdiffusional distances associated with the formation of complex crystal structures, such as perovskite compounds. Synthetic approaches have been geared toward the preparation of multimetal species with cation stoichiometry identical to that of the desired crystalline phase.40 42 83 84... 

Gravimetric analysis utilizes primarily weight measurements and may or may not involve chemical reactions. Titrimetric analysis utilizes both weight and volume measurements and always involves solution chemistry and stoichiometry. 

Dithiothreitol is a more complex ligand than p-mercaptoethanol, and there is a corresponding increase in complexity of the solution chemistry. Two binuclear products (-352, -362 ppm -399, -526 ppm), both of V2L stoichiometry, have been identified. The individual vanadiums of each product are chemically distinct. The NMR studies [37] show that in one of the complexes, both vanadiums have a single coordinated sulfur. In the second complex, one of the two vanadiums has sulfur coordinated, whereas the second of the two has only oxygen in the coordination sphere. Neither of these compounds have been characterized by x-ray diffraction... 

Recall that stoichiometry involves calculating the amounts of reactants and products in chemical reactions. If you know the atoms or ions in a formula or a reaction, you can use stoichiometry to determine the amounts of these atoms or ions that react. Solving stoichiometry problems in solution chemistry involves the same strategies you learned in Unit 2. Calculations involving solutions sometimes require a few additional steps, however. For example, if a precipitate forms, the net ionic equation may be easier to use than the chemical equation. Also, some problems may require you to calculate the amount of a reactant, given the volume and concentration of the solution. 

While studies on the crystal structures of RE(III)-amino acid complexes can give us clear pictures on the ways in which RE(III) ions and the amino acids bond to each other, their solution chemistry, which deals with the reactions in solution, the chemical species formed, their stability, as well as their distribution over certain pH ranges, can help us understand better the in vivo behaviors of RE(III) ions and their complexes with amino acids. Work on the solution chemistry of RE-amino acid complexes has been carried out since the early 1960s [9]. It has been found that the amino acids studied behave very similarly to one another, just as we have learned from their structural chemistry. Mononuclear species with 1 1 and 1 2 (RE L) stoichiometry have been reported for all of the amino acids. In some studies, the presence of mononuclear species with stoichiometry 1 3, dinuclear species with stoichiometry 2 4 and 2 6, in addition to the hydrolyzed species, such as [RE(OH)L]+, [RE(OH)] +, and RE(OH)3(s), have been confirmed [126, 135, 136]. 

Later, Guillamont, et al. [6,7] considered some aspects of the environmental solution chemistry of tracers (complexation, redox reactions) and of solving the specia-tion problems using partition methods. They paid most attention to the concentrations at which the mutual encounters of the tracer entities are somewhat probable. These conditions affect stoichiometry of the reactions and the kinetic laws of the interactions. Below this limit, if two different chemical forms of the tracer were... 

In chemical work, it is important to be able to calculate how much raw material is needed to prepare a certain quantity of products. It is also useful to know if a certain reaction method can prepare more product from a given quantity of material than another reaction method. Analyzing material means finding out how much of each element is present. To do the measurements, parts of the material are often converted to compounds that are easy to separate, and then those compounds are measured. All these measurements involve stoichiometry, tlie science of measuring how much of one thing can be produced from certain amounts of others. Calculations involving stoichiometry are also used in studying the gas laws, solution chemistry, equilibrium, and other topics. 

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