Determining the Formula of a Compound

When a new compound is prepared, one of the first items of interest is its formula. The formula is often determined by taking a weighed sample of the compound and either decomposing it into its component elements or reacting it with oxygen to produce substances such as CO2, H20, and N2, which are then collected and weighed. A device for doing this type of analysis is shown in Fig. 3.5. The results of such analyses provide the mass of each type of element in the compound, which can be used to determine the mass percent of each element present. 

A schematic diagram of the combustion device used to analyze substances for carbon and hydrogen. The sample is burned in the presence of excess oxygen, which converts all of its carbon to carbon dioxide and all of its hydrogen to water. These compounds are collected by absorption using appropriate materials, and their amounts are determined by measuring the increase in weights of the absorbents. 

Remember that this carbon originally came from the 0.1156-gram sample of the unknown compound. Thus the mass percent of carbon in this compound is 

The same procedure can be used to find the mass percent of hydrogen in the unknown compound. We assume that all of the hydrogen present in the 

The unknown compound contains only carbon, hydrogen, and nitrogen. So far we have determined that it is 38.67% carbon and 16.23% hydrogen. The remainder must be nitrogen  

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This factor can now be used to determine the mass of carbon in 0.1638 gram of CO2  

We will see how information of this type can be used to compute the formula of a compound. Suppose a substance has been prepared that is composed of carbon, hydrogen, and nitrogen. When 0.1156 g of this compound is reacted with oxygen, 0.1638 g of carbon dioxide (CO2) and 0.1676 g of water (H2O) are collected. Assuming that all the carbon in the compound is converted to CO2, we can determine the mass of carbon originally present in the 0.1156-g sample. To do this, we must use the fraction (by mass) of carbon in CO2. The molar mass of CO2 is 

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Example 3-15 demonstrates how to use combustion analysis to determine the formula of a compound containing only C and H, and Example shows how combustion analysis is conducted when the compound contains O. 

So far we have started with the chemical formula of a compound and decided on its systematic name. Being able to reverse the process is also important. Often a laboratory procedure describes a compound by name, but the label on the bottle in the lab shows only the formula of the chemical it contains. It is essential that you are able to determine the formula of a compound from its name. In fact, you already know enough about compounds to do this. 

In nature, organic compounds are usually found in incredibly complex mixtures. Destructive distillation of a sample of coal produces hundreds of compounds in easily measurable quantities and thousands of compounds if one wishes to measure trace levels. If a chemist wishes to determine the formula of a compound, it must first be separated from other compounds and rigorously purified. Even today, absolute separation of compounds cannot always be achieved using /in-de-millenrdum techniques. 

Table 7.1 Steps to Determine the Formula of a Compound When Given Percent Composition Data...

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