Empirical and molecular formula

Introduction :

Empirical Formula

The empirical formula for a compound is the simplest ratio of the numbers of atoms of each element present in the compound, e.g. hydrogen peroxide H₂O₂, its empirical formula is HO.

Empirical formula mass of a compound: It refers to the sum of the atomic masses of the elements present in the empirical formula. We can calculate the empirical Formula using the composition of elements in a compound.

Find Molecular Formula

A molecular formula represents the number of atoms of each element in a molecule. Therefore, the multiples of empirical formula gives the molecular formula of a, e.g. hydrogen peroxide H₂O₂. The Molecular Mass (molecular weight) of a compound is calculated with the atomic masses of each element and times with the number of atoms present in a compound.

    Molecular formula mass = n x empirical formula mass

The Molecular formula shows each element by its chemical symbol and indicates the number of atoms of each element found in each molecule of that compound. For example, methane, a small molecule consisting of one carbon atom and four hydrogen atoms, has the chemical formula CH₄. The sugar molecule glucose has six carbon atoms, twelve hydrogen atoms and six oxygen atoms, so its chemical formula is C₆H₁₂O₆.

Examples of Empirical and Molecular Formula:

• Example 1: 

Common sugar: glucose

Ratio of carbon, hydrogen and oxygen are in the ratio of 1:2:1. So, the empirical formula is CH₂O.

Empirical formula mass of glucose: 12.0 + (2 x 1.0) + 16.0 = 30g/mol.

Molar mass or molecular weight of glucose is: 180.16g/mol 

Molecular formula mass = n x empirical formula mass

180.16g/mol = n x 30g/mol

n = 6

Therefore, the molecular formula for a compound is 6 x empirical formula, i.e.., 6 x CH₂O = C₆H₁₂O₆   

• Example 2:  Ethane 

Ratio of carbon, hydrogen and oxygen are in the ratio of 1:2. So,  the empirical formula is CH₃ 

Empirical formula mass is: 12.0 + (3 x 1.0) = 15.0 g/mol

Molar mass or molecular weight of  Ethane is 30.0g/mol 

Molecular formula mass = n x empirical formula mass

30.0 = n x 15.0

n = 2

Therefore, the molecular formula for a compound is 2 x empirical formula, i.e.., 2 x (CH₃) which is C₂H₆

Comparing Molecular and Empirical Formulas

Compound                                     Molecular formula                        Empirical formula

Water                                                 H₂O                                               H₂O

Hydrogen Peroxide                              H₂O₂                                              HO

Glucose                                             C₆H₁₂O₆                                        CH₂O

Methane                                            CH₄                                                CH₄

Ethane                                              C₂H₆                                              CH₃