In chemistry, there are a number of different ways to express a chemical compound. You may use its common name, such as sodium chloride or benzene, or you may chose to express it in a chemical formula. A chemical formula contains more information on a chemical compound than its common name. A chemical formula tells you information about the specific atoms that make up a particular chemical compound.
The two most commonly seen chemical formulas are the empirical formula and the molecular formula. Both formulas tell you what elements make up a specific chemical compound. For example, H2O (both a molecular and an empirical formula) has hydrogen (H) molecules and oxygen (O) molecules. By looking at CaCl (calcium chloride) you will know that it has calcium (Ca) and chlorine (Cl) atoms in it.
Molecular and empirical formulas differ by the numbers in their subscript. The subscript is the ‘2’ in H2O. For a molecular formula, the subscript describes the total number of atoms in a particular molecule. Hexane, a molecule with six atoms of carbon and fourteen atoms of hydrogen is expressed as C6H14 in a molecular formula. For an empirical formula, the subscript describes the ratio of atoms in a particular molecule. Again, hexane with its six carbon atoms and fourteen hydrogen atoms would now be expressed C3H7 because that is the ratio of 6:14.
In high school chemistry textbooks, it is more common to see an empirical formula than a molecular formula. Empirical formulas also describe ionic compounds and macromolecules. Ionic compounds are molecules held together by the electrostatic force that is created when a negatively charged atom reacts with a positively charged one. Macromolecules are large molecules such as nucleic acids and proteins. The empirical formula is especially useful for expressing the formula of macromolecules cleanly. Without the empirical formula the numbers in the subscript have the potential to become enormous.
The empirical formula can be used in physics as well as chemistry. In physics, the empirical formula acts as a mathematical equation. This equation is used to predict observable results in tests on the movements and interactions of atoms.
Summary
1. Empirical and molecular formulas are both chemical formulas.
2. The molecular formula lists all the atoms in a molecule while the empirical formula shows the ratio the number of the atoms in a molecule.
3. Empirical formulas are used to describe ionic compounds and macromolecules.
4. Empirical formulas are used as mathematical equations in physics.