Determining Empirical formula & molecular formula with examples

What is the empirical formula?

Chemical formulas are of two types. The simplest type of formula is the empirical’s formula. It is the simplest whole-number ratio of atoms present in a compound. The empirical formula of a compound is determined by knowing the percentage composition of a compound. However, here we will explain it with simple examples.

The covalent compound silica (sand) has the simplest ratio of 1:2 of silicon and oxygen respectively. Therefore its empirical formula is SiO2. Similarly, glucose has the simplest ratio of 1:2:1 of carbon, hydrogen, and oxygen respectively. Hence its empirical formulae are CH2O.

As discussed earlier, ionic compounds exist in three-dimensional network forms. Each ion is surrounded by oppositely charged ions which is a way to form an electrically neutral compound. Therefore, the simplest unit taken as a representative of an ionic compound is called the formula unit.

It is defined as the simplest whole-number ratio of ions, as present in the ionic compound. In other words, ionic compounds have only empirical formulae. For example, the formula unit of common salt consists of one Na+ and one Cl ion, and its empirical formulae are NaCl. Similarly, the formula unit of potassium bromide is KBr, which is also its empirical formulae.

How to find the molecular formula

Molecules are formed by the combination of atoms. These molecules are represented by a molecular formula that shows the actual number of atoms of each element present in a molecule of that compound. Molecular formulae are derived from the empirical formulas by the following relationship:

relation between molecular formula and empirical formula

For example, the molecular formulae of benzene are C6H6 which is derived from the empirical formulae CH where the value of n is 6.

The molecular formula of a compound may be the same or a multiple of the empirical formula. A few compounds having different empirical and molecular formulae are shown below in the table.

Some Compounds with their Empirical and Molecular Formulae

Compound Empirical formula Molecular formula
Hydrogen peroxide HO H2O2
Benzene CH C6H6
Glucose CH2O C6H12O6

Some compounds may have the same empirical and molecular formulae e.g water (H2O), hydrochloric acid (HCl), etc.

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