Empirical formulas describe the ratio of each type of atom in a substance. The word “empirical” refers to the origins of empirical formulas in observable results; that is, the formulas are determined by experiment. Scientists test the composition of unknown compounds by observing their formation or their interactions with known substances. The ratio of atoms in a compound gives only a rough idea of its nature, but it is a basis for further investigation.
If scientists can create a compound and know the composition of the reagents, or the substances that react, they can determine the empirical formula of the product. They carry out the reaction to produce the product without leaving any unreacted inputs. They also monitor the proportions of the reagents that go into the reaction. It is important that they know the proportion of molecules of the reagents rather than the proportion of mass or volume because these measurements can vary for different molecules. The proportion of reagents tells them the proportion of atoms in the product since all of the molecules they put in are used in the reaction.
Another method of determining empirical formulas is to cause a compound to react with another substance and observe the products of the reaction. Scientists commonly use this method to analyze hydrocarbons, substances which contain only carbon and hydrogen atoms. They burn unknown hydrocarbons and collect the carbon dioxide and water vapor the reaction yields.
When a hydrocarbon burns, it reacts with oxygen; the reaction can be represented as A CfHg + B O2 --> D CO2 + E H2O, where A, B, D and E indicate the proportions of the molecules in the reaction and f and g represent the proportions of carbon and hydrogen in the hydrocarbon. The experimenters measure the mass of carbon dioxide and water vapor. Then they divide these numbers by the molecular weight of the respective compounds. For carbon dioxide, the resulting number is the same as the number of carbon atoms’ but they must multiply the number of water molecules by two to get the number of hydrogen molecules. They find the smallest whole numbers that preserve the proportion of C and H, and these are f and g in the equation: the subscripts of the empirical formula.
Empirical formulas do not indicate the precise structure of a molecule. Acetylene and benzene, for example, are hydrocarbons that contain equal numbers of carbon and hydrogen atoms, so the empirical formula of each is CH. Acetylene’s molecular formula is C2H2, while benzene’s is C6H6. They have vastly different properties despite having the same empirical formula. Acetylene is a highly explosive gas used for cutting and welding; six-carbon benzene rings are the definitive component of aromatic substances and are present in many of the molecules responsible for tastes and smells.