Analía Bellizzi – Chemistry Classes

Ronald Reagan High School

Empirical Formula

Class Notes

Introduction:

There are several ways of showing the formula of a compound:

  • Displayed or Structural formula: Shows all atoms and bonds between them
  • Condensed formula: Show the atoms and which elements are bonded to each but does not show the bonds
  • Molecular formula: Shows only the atoms present in a compound
  • Empirical formula: Minimum ratio of the elements in a molecular formula. It is a simplification of the molecular formula. 

 

Empirical Formula is the minimum whole ratio between the atoms in a molecule expressed in Moles.

Having the % composition as data or the amount of each element in grams in the compound, it is easy to work out the empirical formula:

 

Steps to calculate the empirical formula: 

  1. Find the number of grams of each element, present in the compound. 
  2. Convert the mass of each element to moles using the molar mass from the periodic table. (mass /Ar in grams = # of moles)
  3. Divide each mole value by the smallest number of moles calculated in 2  to get whole number ratios. 

Example 1:

Calculate the empirical formula for a compound that has 19.60g of Iron and 5.60 g of Oxygen

19.60 g Fe ÷ 56 g Fe/mol = 0.35 /0.35 = 1  ==> Fe1

5.60 g O     ÷ 16 g O/mol  = 0.35 / 0.35 = 1 ==> O1

Empirical Formula: FeO

Example 2:

Calculate the empirical formula for a compound that has 69.94%  of Iron and 30.06% of Oxygen.

69.94 g Fe ÷ 56 g Fe/mol = 1.24893 / 1.24893 = 1 

30.06 g O ÷ 16 g O/mol = 1.87875 / 1.24893 = 1.5 

we cannot round numbers that can  be expressed by a fraction.  In this case, we need to multiply BOTH NUMBERS by two to get two WHOLE NUMBERS

Fe = 1            x 2   =  2

O = 1.5         x 2   =   3

 

Videos on how to calculate the empirical and molecular formulas