Wednesday, October 17, 2012

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.
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

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