Ionic formula determination

Ionic compounds are named using the same guidelines used for naming binary molecules, except that the cation name aiways precedes the anion name. Thus, NH4 NO3 is ammonium nitrate, Na2 CO3 is sodium carbonate, and Ca3 (P04)2 is caicium phosphate. The subscripts are not specified in these names because the fixed ionic charges determine the cation-anion ratios unambiguously. Example 3-6 reinforces these guidelines by showing how to construct chemicai formulas from chemical names. 

How do chemists determine how many water molecules are bonded to each ionic formula unit in a hydrate One method is to heat the compound in order to convert it to its anhydrous form. The bonds that join the water molecules to the ionic compound are very weak compared with the strong ionic bonds within the ionic compound. Heating a hydrate usually removes the water molecules, leaving the anhydrous compound behind. In Investigation 6-B, you will heat a hydrate to determine its formula. 

Given the name or formula for an ionic compound, determine the number of moles of each ion in one mole of the compound. 

Formula for an Ionic Compound Determine the formula for the ionic compound formed from potassium and oxygen. 

What is ionic mass The ionic mass of an ion takes into account the mass of an electron (0.000548 Da = 0.548 mDa mDa is also referred to as millimass units, mmu) that is removed or added during the formation of the ion (remember that a proton is a hydrogen atom, H isotope, after removal of an electron). This small mass effect is frequently ignored when masses are determined experimentally and compared with calculated values. However, as the accuracy and precision of accurate mass measurements have improved, the mass attributable to an electron has become of relevance to empirical formula determinations. The mass of an electron represents 1 part per million at 500 Da (-0.5 mDa), which is a significant error in miz determinations for instruments designed to obtain mass accuracies at the mDa level or less. 

By this method, we are able to apply the formula [4.82] to a broader range of ionic strengths. However, the values thus determined for a still need to be acceptable in terms of the physical meaning. Indeed, certain experiments yield very low values of the radius a - sometimes zero, and sometimes even negative values are required. The true value of a must be no lower than the ionic radius determined in a crystal of a corresponding salt. 

A major task of chemical analysis is to determine the formulas of compounds. The formula found by the approach described here is the simplest formula, which gives the simplest whole-number ratio of the atoms present. For an ionic compound, the simplest formula is ordinarily the only one that can be written (e.g., CaCl2, Cr203). For a molecular compound, the molecular formula is a whole-number multiple of the simplest formula, where that number may be 1,2. 

First check to see whether the compounds are ionic or molecular. Many compounds that contain a metal are ionic. Write the symbol of the metal first, followed by the symbol of the nonmetal. The charges on the ions are determined as shown in Examples C.l and C.2. Subscripts are chosen to balance charges. Compounds of two nonmetals are normally molecular. Write their formulas by listing the symbols of the elements in the same order as in the name, with subscripts corresponding to the Greek prefixes used. 

We learned to write formulas of ionic compounds in Chaps. 5 and 6. We balanced the charges to determine the number of each ion to use in the formula. We could not do the same thing for atoms of elements in covalent compounds, because in these compounds the atoms do not have charges. In order to overcome this difficulty, we define oxidation numbers, also called oxidation states. 

EXAMPLE 5.4. With the aid of the periodic table, use electron dot notation to determine the formula of the ionic compound formed between potassium and sulfur. 

In addition to [Hg( -toluene)2-(GaCI/ )2],168 other mercury-arene complexes of general formula [I Ig( /2-arene)2-(AlCUy have been prepared.169 These include the bis(toluene), bis(o-xylene), and bis(l,2,3-trimethylbenzene) complexes 159, 160, and 161, respectively, whose structures have all been determined (Figure 8). While the arene in 159 and 161 is coordinated in an asymmetrical -fashion, the /2-1,2,3-trim ethylbenzene ligands of 160 form two nearly equal Hg-C bonds of 2.45 and 2.46 A. DFT calculations show that the Hg-arene interactions are mostly ionic. 

Some values for the enthalpy of formation of Schottky defects in alkali halides of formula MX that adopt the sodium chloride structure are given in Table 2.1. The experimental determination of these values (obtained mostly from diffusion or ionic conductivity data (Chapters 5 and 6) is not easy, and there is a large scatter of values in the literature. The most reliable data are for the easily purified alkali halides. Currently, values for defect formation energies are more often obtained from calculations (Section 2.10). 

Next, rule 10 is used for determining all oxidation numbers of all elements that rules 1 through 8 do not cover (this must be all but one element in a formula), and then assigning the remaining elements oxidation numbers, knowing that the total must add up to either zero or the ionic charge. 

The MS analysis using ESI was applied for the determination of an unknown surfactant compound contained in an extract of a shampoo formulation [44]. MS leading to sequential product ions helped to identify the constituents. The MS4 experiments together with other spectral observations confirmed the hypothesis that the unknown compound was a N-( 2-aminoethyl) fatty acid amide with the general formula R-C(0)-NH(CH2-CH2-N)R/R,/. An authentic sample of the proposed laury ampho mono acetate (LAMA) (R = -CH2-CH2-OH and R" = -CH2-CH2-COOH) that was available led to the same [M + H]+ parent ion at m/z 345. The fragmentation that could be observed under ESI-FIA-MS-MS(+) conditions resulted in an intensive examination of amide surfactants. However, only two of them—lauryl diethanol amide ([M + H]+ m/z 288), a non-ionic surfactant and laurylamido-(3-propyl betaine ([M + H]+ m/z 343)—... 

Once the composition of the aqueous solution phase has been determined, the activity of an electrolyte having the same chemical formula as the assumed precipitate can be calculated (11,12). This calculation may utilize either mean ionic activity coefficients and total concentrations of the ions in the electrolyte, or single-ion activity coefficients and free-species concentrations of the ions in the electrolyte (11). If the latter approach is used, the computed electrolyte activity is termed an ion-activity product (12). Regardless of which approach is adopted, the calculated electrolyte activity is compared to the solubility product constant of the assumed precipitate as a test for the existence of the solid phase. If the calculated ion-activity product is smaller than the candidate solubility product constant, the corresponding solid phase is concluded not to have formed in the time period of the solubility measurements. Ihis judgment must be tempered, of course, in light of the precision with which both electrolyte activities and solubility product constants can be determined (12). 

The crisscross rule can help determine the formula of an ionic compound. 

To use Equation 2 to determine s electron density diflFerences, it must be "calibrated —i.e., source-absorber or absorber-absorber combinations must be found for which the 5 electron density diflFerence is known. The most common method for calibrating the isomeric shift formula is to measure isomeric shifts for absorbers with diflFerent numbers of outer shell 5 electrons—e.g., by using compounds with the absorbing atoms in different valence states. The accuracy of this method depends on how much is known about the chemical bonds in suitably chosen absorber compounds, in particular about their ionicity and their hybridization. t/ (0) 2 can be obtained for an outer 5 electron from the Fermi-Segre formula or preferably from Hartree-Fock calculations. 

Cations and anions combine in very predictable ways within ionic compounds, always acting to neutralize overall charge. Therefore, the name of an ionic compound implies more than just the identity of the atoms that make it up. It also helps you determine the correct chemical formula, which tells you the ratio in which the elements combine. Consider these two examples, both of which involve lithium ... 

AB2 structures. Fluorides and oxides of the formula AB2, which are distinctly ionic, crystallize in structures determined by size considerations. As in the case of AB structures, it is the coordination geometry of anions around the cation that determines the structural arrangement. The coordination may be 8-, 6-, or 4-fold, fixing the corresponding anion coordination numbers to 4,3 or 2. We thus have the following structures for ionic AB2 compounds fluorite (8 4), rutile (6 3) and silica (4 2) and these structures are shown in Fig. 1.7. 

The poly(metal)silazanes 62-77 are isolated usually as colorless crystals. They are unstable with respect to water and oxygen and are soluble in nonpolar solvents (with the exception of compound 77), The formulas drawn for 64-67 are merely formal and do not imply that these substances have a considerable ionic character. They are soluble in benzene and are monomo-lecular as found by cryoscopic molecular weight determinations. The H NMR spectra of 64 and 65 (64) demonstrate that the metal atoms are coordinated by methyl groups, the most reasonable structure being depicted in the formulas 64 and 65. The trigonal-bipyramidal arrangement of the N2SiAlM... 

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