Predicting Formulas from Percentage Compositions

You should be able to predict the formula of a compound on the basis of a given data of percentage compositions. Study the next example to understand how it is done. 

A carbon compound contains 27.27% carbon and 72.73% oxygen by mass. Predict the simplest ratio or formula of the compound. 

The best way to approach this problem is to consider that we have 100 grams of this compound. Logically it should contain 27.27 grams of carbon and 72.73 

Divide every number of moles with the smallest number of moles that you got in Step 1. Here the smaller one is 2.2725. So divide the number of moles of carbon atoms and the number of moles of oxygen atoms by 2.2725. That will give you the simplest ratio between them. 

The easiest way to calculate this is to find the percentage composition of sulfur. Then, use that percentage to find the mass of sulfur in the given amount of substance. 

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In the previous Practice Problems, you used mass data to calculate percentage composition. This skill is useful for interpreting experimental data when the chemical formula is unknown. Often, however, the percentage composition is calculated from a known chemical formula. This is useful when you are interested in extracting a certain element from a compound. For example, many metals, such as iron and mercury, exist in mineral form. Mercury is most often found in nature as mercury(II) sulfide, HgS. Knowing the percentage composition of HgS helps a metallurgist predict the mass of mercury that can be extracted from a sample of HgS. 

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