Percent Composition How-To

In chemistry, percent composition is an important tool used to solve problems, especially those relating to empirical formulas. Percent composition, when used correctly, determines what amount of a substance is in a chemical formula. In order to find the percent composition, you need no more than a title and the atomic mass of the elements in the substance. Lets take carbon dioxide (CO2) as an example.

First, one must find the atomic mass of the elements in the formula. The mass (equivalent of one mole) of carbon, is 12.01, and the mass of oxygen is 16.00. Then, those masses must be multiplied by the number of atoms in the compound (CO2), so 12.01×1= 12.01 and 16.00×2= 32.00. After that, the resulting masses (12.01 and 32.00) must be added to find the total mass of the compound, resulting in 44.01. Using the total mass of the compound, the weight of an element in the compound must be divided by the total mass, for example, 12.01g C/44.01 and 32.00g O/44.01, and then multiplied by 100 to find the end result of percent composition. So carbon dioxide (CO2), is 27.29 percent carbon and 72.71 percent oxygen. It is important to use significant figures throughout to make calculations precise, and then if the number needs to be rounded later, it is simple to do.


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