Empirical formulas describe the ratio of atoms in a substance, determined by experiment. Scientists use reactions to determine the empirical formula of a product, and can also analyze hydrocarbons by burning them and measuring the resulting carbon dioxide and water vapor. Empirical formulas do not indicate the precise structure of a molecule.
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 interactions with known substances. The ratio of atoms in a compound gives only a rough idea of its nature but is a basis for further investigation.
If scientists can create a compound and know the composition of the reactants, or substances that react, they can determine the empirical formula of the product. They run the reaction to produce the product leaving no unreacted inputs. They also monitor the proportions of reactants going into the reaction. It is important that they know the proportion of molecules of the reactants rather than the proportion by mass or volume because these measurements can vary for different molecules. The proportion of reactants tells them the proportion of atoms in the product as all the molecules they put in are used up in the reaction.
Another method of determining empirical formulas is to react a compound with another substance and observe the products of the reaction. Scientists commonly use this method to analyze hydrocarbons, substances that contain only carbon and hydrogen atoms. They burn unknown hydrocarbons and collect the carbon dioxide and water vapor that the reaction produces.
When a hydrocarbon burns, it reacts with oxygen; the reaction can be represented as A CfHg + BO2 –> 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. 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 equals 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 integers 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 for each is CH. The molecular formula of acetylene is C2H2, while that of benzene is C6H6. They have very different properties despite having the same empirical formula. Acetylene is a highly explosive gas used for cutting and welding; benzene rings with six carbon atoms are the definitive component of aromatic substances and are present in many of the molecules responsible for flavors and odours.
Protect your devices with Threat Protection by NordVPN
