Empirical formula of a substance represents mass of the substance equal to the formula mass of the substance. The formula mass of the substance is equal to the sum of the atomic masses of all the atoms present in the empirical formula.
In the case of ionic compounds, or giant covalent compounds having network structure (e.g., diamond) there is no discrete molecule. So it is meaningless to talk of a molecule, and molecular formula for such substances. An ionic or a giant covalent compound is described by a simple formula commonly called as the empirical formula. An empirical formula is also called as the stiochiometric formula.
Empirical formula (or stiochiometric formula) of a substance is the simplest formula which gives the lowest whole-number ratio between the number of atoms of different elements present in the substance.
For example, crystalline (or solid) sodium chloride is a three-dimensional structure containing sodium (Na+) and chloride (Cl–) ions. So, sodium chloride may be represented by a formula (Na+ Cl–) n, where n is a large number. The actual number of Na+ ions and Cl– ions in a sample of sodium chloride depends upon the size of the sample. But , in all sample, small or big, the ratio between the number of Na+ ions and Cl- ions is always 1:1. So, the simplest formula for sodium chloride is Na+Cl–, or only as NaCl. Therefore, the empirical formula (or stiochiometric formula) of sodium chloride is NaCl.
Another example will probably make it clearer. A covalent compound has a molecular formula C6H6, i.e., there are six carbon atoms and six hydrogen atoms in its molecule. The lowest whole number ratio between the number of carbon and hydrogen atom is 1:1, (6:6 can be simplified to 1:1). Therefore, the empirical formula of compound having molecular formula of C6H6 is CHALLENGE.
An empirical formula provides the following information.
(i) Empirical formula of a compound gives us the names of all the elements present in the compound.
(ii) Empirical formula of a compound gives the simplest whole-number ratio between the numbers of atoms of all the elements present in the compound.
(iii) Empirical formula of a substance represents mass of the substance equal to the formula mass of the substance. The formula mass of the substance is equal to the sum of the atomic masses of all the atoms present in the empirical formula. For example , for sodium chloride(NaCl), the formula mass is equal to sum of the atomic masses of Na and Cl viz. equal to (23 u+ 35.5 u) = 58.5u.