Empirical Formula | How to Determine Empirical Formula

Definition:

A formula which indicates the simplest whole number ratio of atoms of different elements present in a compound is called an empirical formula.

How to Determine Empirical Formula:

1:  First, calculate the percentage of each element present in a compound.
2:  Divide the percentage of each element by its atomic mass to get the number of moles of each element.
3:  Divide number of moles of each element by least value to get simple whole number ratio of atoms.
4:  If atomic ratios are in fractions, multiply these by a suitable number to get simple whole number ratio.
5:  Write atomic ratios of elements below their symbols to get the empirical formula.
To understand let's take an example

Example:

Ascorbic acid (vitamin C) contains 40.92 % carbon, 4.58% hydrogen and 54.5 % of oxygen by mass. Now we calculate or determine the empirical formula from the above data.

                     mass of C = 40.92%

                     mass of H = 4.58%

                     mass of O = 54.5%

Step 1:

Dividing the percentages of elements by their atomic masses to get the moles of each element.
            Moles of C   = 40.92/ 12.01   =   3.407

            Moles of  H  =  4.58/1.00       =   4.54
            Moles of  O  =  54.5/16          = 3.406
Step 2: To get simple shole number ratio of atoms, divide above numbers of moles by the least number which is 3.406
            C   =     30407/3.406    = 1
            H   =    4.54/3.406        = 1.33
            O   =     3.406/3.406     = 1

Step 3: Convert these values into whole number by multiplying with 3.
         C:H:O         =        1:1.33:1
                             =       3(1:1.33:1)   =    3:4:3

Step 4: Atomic ratios of elements give the empirical formula for ascorbic acid. 
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