Elemental Analysis Determining Empirical and Molecular Formulas

Elemental Analysis Determining Empirical and Molecular Formulas 

When a new molecule is synthesized, an elemental analysis is routinely performed to help verify its identity. This test, which measures the mass percentage of each element in the compound, is also Ifequendy done as part of the process of identifying any substance whose composition is unknown. The mass percentages describe the compound s composition, and so they must be related to its chemical formula. But the data obtained from elemental analysis describe the composition in terms of the mass of each element, whereas the formula describes the composition in terms of the number of atoms of each element. So these are two different representations of very similar information, and the molar masses of the elements provide a connection between them. The process of obtaining the empirical formula of a compound from its percent composition by mass is best illustrated by an example. 

The explosive known as RDX contains 16.22% carbon, 2.72% hydrogen, 37.84% nitrogen, and 43.22% oxygen by mass. Determine the empirical formula of the compound. 

The empirical formula is based on the mole ratios among the elements in the compound, and the data given are in terms of mass. Molar mass provides the link between mass and moles, as usual. We can begin by choosing a convenient mass of the compound—usually 100 g—and use the percentages given to find the mass of each element in that sample. Then we will convert those masses into numbers of moles of each element. The ratios between those numbers of moles must be the same for any sample of the compound. Finally, we will need to convert those ratios into whole numbers to write the empirical formula. 

Consider a 100-g sample of RDX. From the percentages given, that sample will contain 16.22g C, 2.72g H, 37.84g N, and 43.22g O. We will convert each of 

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