Using Gravimetric Factors

As stated earlier, gravimetric factors are used to convert the weight of one chemical to the weight of another, as in the example cited at the beginning of Section 3.6.3. Below is another example of such a conversion. 

How many grams of copper(II) sulfate pentahydrate are required to prepare a solution that has the 

The formula of copper(II) sulfate pentahydrate is CuS04-5H20 and its formula weight is 249.686 g/mol. 

Substance sought CuS04-5H20 Substance known Cu 

In the case in which the analyte participates in a chemical reaction, the product of which is weighed, the weight of this product must be converted to the weight of the analyte before the percent can be calculated. 

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The weight of the precipitate after filtering and drying can then be measured free of any influence from the NaCl and converted back to the weight of the analyte with the use of a gravimetric factor (see the next section) and its percent in the sample calculated. Examples are given in Section 3.6.4. 

A gravimetric factor is a number used to convert, by multiplication, the weight of one chemical to the weight of another. Such a conversion can be very useful in an analytical laboratory. For example, if a recipe for a solution of iron calls for 55 g of FeCl3 but a technician finds only iron wire on the chemical shelf, he or she would want to know how much iron metal is equivalent to 55 g of FeCl3 so that he or she could prepare the solution with the iron wire instead and have the same weight of iron in either case. In one formula unit of FeCl3, there is one atom of Fe, so the fraction of iron(III) chloride that is iron metal is calculated as follows ... 

This fraction is therefore a gravimetric factor because it is used to convert the weight of FeCl3 to the weight of Fe, as shown. 

This can be done with the use of a gravimetric factor. Examples include the sulfate and iron determinations in the experiments at the end of this chapter (see Experiments 6 and 7). 

What is the gravimetric factor that must be used in each of the following experiments ... 

The same mole ratio, a/d, can also be found by simply balancing the common element in the formulas of A and D. Thus, the ratio, a/d, is the same as the ratio QS/QK seen in the equation for a gravimetric factor derived in Section 3.6.3 (Equation (3.12)) and is used to convert the weight of one substance to the weight of another, just as we described in Chapter 3 was the purpose of the gravimetric factor. Thus the concept of a gravimetric factor is based on stoichiometry. 

For a reaction in which the stoichiometric relation between analyte and product is not 1 1, we must use the correct stoichiometry in formulating the gravimetric factor. For example, an unknown containing Mg2+ (atomic mass = 24.305 0) can be analyzed gravimetrically to produce magnesium pyrophosphate (Mg2P207. FM 222.553). The gravimetric factor would be... 

The atomic weights used in computing formula weights and volumetric and gravimetric factors stated in tests and assays are those recommended in 1991 by the IUPAC Commission on Isotopic Abundances and Atomic Weights. 

Considering the low Rp, the absence of new peaks on the second Fourier map, which may correspond to additional atoms, and the fact that the contents of the unit cell matches that established from the gravimetric density of the material, we conclude that all atoms in the unit cell of LaNi4,85Sno.i5 have been located. It makes no sense to proceed with the least squares refinement of atomic parameters using structure factors determined from Le Bail s extraction, and the refinement of the crystal structure should be completed using the Rietveld technique (see Chapter 7). The coordinates and possible distribution of atoms are listed in Table 6.8 and the crystal structure of the material is illustrated in Figure 6.14. 

Orthophosphate (P04 ) is determined by weighing as ammonium phospho-molybdate, (NH4)P04 I2M0O3. Calculate the percent P in the sample and the percent P2O5 if 1.1682 g precipitate (ppt) were obtained from a 0.2711-g sample. Perform the % P calculation using the gravimetric factor and just using dimensional analysis. 

Sodium oxide is not stable in air, so we must use a batch component such as Na2COj, which yields Na O after decomposition. It is necessary to multiply the desired quantity of Na20 by the gravimetric factor for Na2C03 (1.71), to obtain the weight of Na2C03 (35.05 g) to be used to yield the desired 20.5 g of Na20. 

Using the atomic mass spread sheets described in Example 1.2, calculate and record suitably on the spreadsheet the gravimetric factors for the following ... 

For analysis of solutions containing adrenaline the net error due to using the rotatory and gravimetric factors applicable to adrenaline is less than 1 per cent and may be ignored. 

Ash and Inorganic Constituents. Ash may be measured gravimetdcaHy by incineration in the presence of sulfudc acid or, more conveniendy, by conductivity measurement. The gravimetric result is called the sulfated ash. The older carbonate ash method is no longer in use. Ash content of sugar and sugar products is approximated by solution conductivity measurements using standardized procedures and conversion factors. 

All reagents and solvents that are used to prepare the sample for analysis should be ultrapure to prevent contamination of the sample with impurities. Plastic ware should be avoided since these materials may contain ultratrace elements that can be leached into the analyte solutions. Chemically cleaned glassware is recommended for all sample preparation procedures. Liquid samples can be analyzed directly or after dilution when the concentrations are too high. Remember, all analytical errors are multiplied by dilution factors therefore, using atomic spectroscopy to determine high concentrations of elements may be less accurate than classical gravimetric methods. 

The resin supply system should be designed to take advantage of the raw materials in the lowest cost and most effective form. Additives tend to be more expensive than the base resin. Gravimetric rather than volumetric supply of the material is more conducive to minimizing the use of the more expensive feedstock components. The ability of the equipment to utilize reliably 100% of in-plant regrind, additive concentrates, and recycled materials is one of the most important factors to be considered. 

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