Stoichiometric formula

Boron carbide is a non-metallic covalent material with the theoretical stoichiometric formula, B4C. Stoichiometry, however, is rarely achieved and the compound is usually boron rich. It has a rhombohedral structure with a low density and a high melting point. It is extremely hard and has excellent nuclear properties. Its characteristics are summarized in Table 9.2. 

In cases where the antisite defects are balanced, such as a Ga atom on an As site balanced by an As atom on a Ga site, the composition of the compound is unaltered. In cases where this is not so, the composition of the material will drift away from the stoichiometric formula unless a population of compensating defects is also present. For example, the alloy FeAl contains antisite defects consisting of iron atoms on aluminum sites without a balancing population of aluminum atoms on iron sites. The composition will be iron rich unless compensating defects such as A1 interstitials or Fe vacancies are also present in numbers sufficient to restore the stoichiometry. Experiments show that iron vacancies (VFe) are the compensating defects when the composition is maintained at FeAl. 

Raunkjaer et al. (1994) have modified methods for the determination of carbohydrates, proteins and lipids to be applied for wastewater. Henze et al. (1995b) estimate the average composition of these three organic fractions in wastewater and propose a corresponding conversion to COD units (Table 3.3). The table also includes stoichiometric formulas for carbohydrate and protein, proposed by Kalyuzhnyi et al. (2000). 

Table 5.4 lists analyses of some natural specimens. In some instances, there is a silicon deficiency with respect to the stoichiometric formula and diadochy of Ai3+ jj4+ pg3+ Qjj tetrahedral sites (cases 1, 2, 3, and 4). In other cases, the amount of sihcon is near the theoretical value and only divalent cations are virtually present in the M and Ml positions (cases 5 and 6). 

If Ni moles of component H2O mix with N2 moles of an anhydrous melt whose stoichiometric formula contains r moles of oxygen (i.e., r = 8 for NaAlSi30g, r = 2 for Si02, and so on), because one mole of O is necessary to form two moles of OH hydroxyls, we obtain... 

Formula FeS MW 87.911 the pure compound is nonstoichiometric, deficient in iron the stoichiometric formula may be Fe2" o.86S2 ,... 

To minimize any ambiguity in interpretation, we have prepared modified polyethylenimines in which all the nitrogens have been quaternized, with various apolar groups, so that they cannot function in a nucleophilic mechanism. Some of the derivatives prepared may be represented by the stoichiometric formulas... 

The DME adduct of cyclopentadienylsodium is a white powder soluble in tetrahydrofuran (THF) and insoluble in toluene or diethyl ether. It darkens when exposed to air, but is stable enough to be easily transferred under N2 from one Schlenk tube to another. This solid reagent can easily be weighted and used stoichiometrically formula wt [NaC5H5 DME] = 178.2. 

As the starting materials react, all of these compounds immediately precipitate from the reaction mixture and their metahligand stoichiometries are controlled by the lack of solubility. It was only possible to determine the structure of complex 102, which crystallizes with acetone and has a stoichiometric formula [Cu2(bpzqnx)2 (Me2 CO)] (BF4) 2 (Me2CO)2 . As explained below, the structure consists of unsaturated CU2L2 units, half of which contain the copper center also bonded to acetone molecules. 

It is to be borne in mind that the stoichiometric formula of a compound may not indicate uniquely the ligancy of its complexes. Thus Hoard and Martin91 have shown that the crystal K HNbOF7 does not... 

On being heated rubidium superoxide loses one-fourth of its oxygen, to form a substance with the stoichiometric formula RbjO. This substance, originally supposed to contain the ion Og, with the structure... 

FIGURE 1. Possible structural formulas for a molecule with a C3-ring and the stoichiometric formula... 

Systematic names and stoichiometric formulae are intended to convey as much information as possible in the smallest space concerning the composition and nature of a substance. The internationally-agreed (though not universally-obeyed) rules for the naming of inorganic compounds are set out briefly in most inorganic texts, and in The Handbook of Chemistry and Physics they need not be repeated here. 

The student may well feel that the need to name inorganic compounds is a relic of the past, when stoichiometric formulae were often uncertain, and that a formula will convey a more concise specification of a substance. This attitude is not without merit, especially where we are dealing with rather complex substances whose systematic names are hopelessly clumsy and lengthy. For example, the important substance rendered as IrCl(CO)(PPh3)2 on the printed page has the systematic name trans-chlorocarbonylbis(triphenylphosphine)iridium(I). Although hundreds of papers have been published on the reactions of this compound and tens of thousands of students have heard of it, the systematic name is rarely used. In speech, and often in print, it is given the trivial name Vaska s compound . In any research laboratory, you may hear talk of Judy s compound , or Ralph s anion . 

In the case of the reaction of Mn(CO)5CH3 shown above, isotopic labelling studies (with 14CO) show that the CO molecule inserted into the Mn-CH3 bond is onp of the five CO groups already coordinated, and not the external CO shown in the stoichiometric formula. It appears that an equilibrium is set up between the Mn(CO)5CH3 and a five-coordinate, 16-electron (i.e. coordinatively-unsaturated) acyl complex ... 

Dichromium pentafluoride is an idealized formula, since the compound is reported (55) to have a composition in the range CrF2.40-2.45-However, the structure derived using X-ray data, from a single crystal grown by high-temperature melt techniques, is based on the stoichiometric formula, with some vacant anion sites (53). The compound is monoclinic, a = 7.773(5), b = 7.540(5), c = 7.440(5) A, j3 = 124.25(1)°, space group C2/c. 

The stmcture of fluorite, CaF2, may be regarded as being composed of cubic closest packed Ca2+ cations with all the tetrahedral interstices occupied by F-anions, as shown in Fig. 10.1.6. The stoichiometric formula is consistent with the fact that the number of tetrahedral holes is twice that of the number of closest packed atoms. 

A large number of metal borides have been prepared and characterized. Several hundred binary metal borides M Bj, are known. With increasing boron content, the number of B-B bonds increases. In this manner, isolated B atoms, B-B pairs, fragments of boron chains, single chains, double chains, branched chains, and hexagonal networks are formed, as illustrated in Fig. 13.3.1. Table 13.3.1 summarizes the stoichiometric formulas and structures of metal borides. 

Table 13.3.1. Stoichiometric formulas and structure of metal borides...

In principle, the negatively charged, presumably planar network I can be combined with one molar equivalent of tetraalkylammonium ion IGN"1" of the right size as interlayer template to yield a crystalline inclusion compound of stoichiometric formula (IGN+) I C(NIG ) ICO2 that is reminiscent of the graphite intercalates. Anionic network n, on the other hand, needs twice as many monovalent cations for charge balance, and furthermore possesses honeycomb-like host cavities of diameter 700 pm that must be filled by... 

The compounds discussed above are not nonstoichiometric in the strict sense of the word, since they form well defined, ordered phases which can be assigned precise stoichiometric formulas. Accordingly, they are not berthollides. They may, however, be regarded as nonstoichiometric in the wider sense of the term which has become current, since fractional valence numbers must be assigned to the metal atoms and the ordered structures which they exhibit constitute a preferred alternative to the formation of disordered phases of variable composition. 

The cubic phase in each of the Group IVA hydrides has the fluorite structure, so that the hydrides should have the stoichiometric formula, MH2, when all the normal lattice positions are occupied. However, these structures become unstable before reaching the stoichiometric composition, and undergo a phase transition to a tetragonal structure. The room temperature homogeneity ranges of these hydrides are shown in Table II. Titanium hydride also forms a tetragonal phase... 

The body-centered cubic Group VA metals dissolve a considerable amount of hydrogen before distorting to either a b.c. tetragonal or orthorhombic structure. These behave somewhat more like solid solutions of hydrogen in metal than do the more definite hydrides previously discussed. They appear to have the stoichiometric formula MH, but this has never been reported, the maximum being about... 

The ideal stoichiometric formulas for the hydrates prepared with the aid of H2S are, for example, 6C12-2H2S-46H20, or 8C3H816H2S136H20 for types I and II hydrates, respectively. They are referred to as double hydrates. It is also possible to occupy the voids with mixtures of appropriate gases such as the mixture of methane, ethane, and propane, which occurs in natural gas, to yield mixed hydrates. 

Sulfur-Containing Systems. Irradiation of S02, SF4, or SFe gives no observed volatile products. Sulfuryl fluoride on the other hand was found to produce a number of products these include 02, OF2, SFe, SOF4, and some species with the probable stoichiometric formula F2S04 (the compound FS02-00F is known). 

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