Derivation of Empirical Formulas

While it is clear that percent composition can be derived from formulas, the reverse is also true. Empirical formulas, or formulas in which the subscripts are in their lowest terms (e.g., acetylene [CjHj] has an empirical formula of CH), can be derived from percent composition data. Knowing what elements are in the compound and having some data about the relative amounts of these elements in the compound, we can determine the subscripts and the empirical formula. 

As we learned in Chapter 1, the subscripts represent the number of atoms of each element in the formula. In the formula HjO, for example, we know that 

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Compounds of empirical formula M(OR) and LxM(OR)y, where R is an alkyl or aryl group, Lx represents a combination of other ligands, which may be either neutral or anionic, and n and y are integers >1, are the subjects of this chapter. Excluded from consideration are catecholates and semiquinones, and chelated ligands containing alkoxy or phenoxy functionalities, e.g. compounds derived from the ligand formed by deprotonation of 2-hydroxypyridine. 

Just as we can derive the empirical formula of a substance from its percent composition, we can also calculate the percent composition of a substance from its empirical (or molecular) formula. The strategies for the two kinds of calculations are exactly opposite. Aspirin, for example, has the molecular formula C9H8O4 and thus has a CH 0 mole ratio of 9 8 4. We can convert this mole ratio into a mass ratio, and thus into percent composition, by carrying out mole-to-gram conversions. 

Derive the empirical formula of a hydrocarbon that on analysis gave the following percentage composition C = 85.63% and H = 14.37%. 

Derive the empirical formulas of the minerals that have the following compositions (a) ZnSC>4 = 56.14%, H2O =... 

Derive the empirical formula of borazine. Determine the number of moles of each element, mole B = (40.29g ) x ( / gl ) = 3-727moles B... 

The combustion of 3.795 mg of liquid B, which contains only C, H, and O, with excess oxygen gave 9.708 mg of CO2 and 3.969 mg of H2O. In a molar mass determination, 0.205 g of B vaporized at 1.00 atm and 200.0°C and occupied a volume of 89.8 mL. Derive the empirical formula, molar mass, and molecular formula of B and draw three plausible stmc-tures. 

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