Chemical Formula and Percentage Composition

The empirical formula H2O specifies that for every atom of oxygen in water, there are two atoms of hydrogen. Equivalently, one mole of H2O contains two moles of hydrogen atoms and one mole of oxygen atoms. The number of atoms and the number of moles of each element are present in the same ratio, namely, 2 1. The empirical formula for a substance is clearly related to the percentage composition by mass of that substance. This connection can be used in various ways. 

The empirical formula of a compound can be simply related to the mass percentage of its constituent elements using the mole concept. For example, the empirical formula for ethylene (molecular formula C2H4) is CH2. Its composition by mass is calculated from the masses of carbon and hydrogen in 1 mol of CH2 formula units  

Determination of Empirieal Formula from Measured Mass Composition 

We can reverse the procedure just described and determine the empirical formula from the elemental analysis of a compound, as illustrated by Example 2.3. 

00-g sample of a dry-cleaning fluid was analyzed and found to contain 10.80 g carbon, 1.36 g hydrogen, and 47.84 g chlorine. Determine the empirical formula of the compound using a table of atomic masses. 

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