Calculations empirical formulas

Nominal ion mass. The mass of an ion with a given empirical formula calculated using the integer mass numbers of the most abundant isotope of each element, e.g., C = 12, H = 1, O = 16. 

Test your empirical formula calculations at www. brightredbooks.net... 

The approximate molar mass, calculated from the gas density data, is 89 g/mol. The empirical formula, calculated from the percentage composition data, is C2H3O with the empirical formula unit mass of 43.0. The exact molar mass must be (2)(43) = 86.0 g/mol since this is the only multiple of 43.0 (whole-number multiple) reasonably close to the approximate molecular formula of 89 g/mol. The molecule must be the equivalent of 2 empirical formulas CqHgO. 

Step 4 If necessary, multiply all the numbers of moles by the same small integer to clear fractions. Round off the result to an integer only when the number of moles is within 1% of the integer. Always use at least three significant digits in empirical formula calculations otherwise, rounding errors may produce an incorrect empirical formula. 

Note that we do not use the molecular mass of an oxygen molecule (or any other diatomic molecule) in empirical formula calculations because we are interested in the mole ratio involving oxygen atoms. Molecular oxygen, O2, has nothing to do with empirical formula calculations. 

To determine an empirical formula, calculate the number of moles of each element in a sample (arbitrarily choose 100 g if percentages are given), divide each of them by the smallest number of moles, and then multiply by some small integer if necessary to get integral numbers of moles. 

Most abundant ion mass the mass that corresponds to the most abundant peak in the isotopic cluster of the ion of a given empirical formula. Average mass the mass of an ion for a given empirical formula calculated with the atomic weight of each element (e.g., C = 12.01115 and H = 1.00797) that is, the average of the isotopic masses of each element, weighted for isotopic abundance. The average mass represents the centroid of the distribution of the isotopic peaks of the molecular ion and is used by chemists in stoichiometric calculations. 

---