Calculation of Empirical Formulas

In each case below, the molecular formula for a compound is given. Determine the empirical formula for each compound. 

As we said in the previous section, one of the most important things we can learn about a new compound is its chemical formula. To calculate the empirical formula of a compound, we first determine the relative masses of the various elements that are present. 

One way to do this is to measure the masses of elements that react to form the compound. For example, suppose we weigh out 0.2636 g of pure nickel metal into a aucible and heat this metal in the air so that the nickel can react with oxygen to form a nickel oxide compound. After the sample has cooled, we weigh it again and find its mass to be 0.3354 g. The gain in mass is due to the oxygen that reacts with the nickel to form the oxide. Therefore, the mass of oxygen present in the compound is the total mass of the product minus the mass of the nickel  

Mass of oxygen that reacted with the nickel 

Note that the mass of nickel present in the compound is the nickel metal originally weighed out. So we know that the nickel oxide contains 0.2636 g of nickel and 0.0718 g of oxygen. What is the empirical formula of this compound  

To answer this question we must convert the masses to numbers of atoms, using atomic masses  

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HRMS is expensive and complex, and sensitivity is sacrificed for resolution. Rarely is enough of an aquatic pollutant, especially a pesticide residue, available for analysis by HRMS. However, advancements are rapid GC-HRMS analysis with on-line computer calculation of empirical formulae of each ion of each GC eluant (21) is now possible. Kearns (19) has discussed computer analysis of high resolution mass spectra. 

Calculation of empirical formulae from percentage data... 

Calculation of Empirical Formula from Percent Composition... 

Summarize in steps the calculation of empirical formula from the percentage composition of a compound. 

High mass resolution of ToF-SIMS instruments (10 to 10 amu) enables exact mass measurements, calculation of empirical formulae and more reliable unknown peak identification [130]. Accurate mass determination by ToF-SIMS is very simple because no special calibration procedures or calibration standards are required. The accuracy is about 10 ppm for atomic species and for molecules in the low mass range. For organic molecules in the high mass range an accuracy better than 5 ppm was obtained. In cases where the mass resolution is not sufficient, mass spectral fragmentation patterns may be compared to library spectra for identification. 

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