Empirical Formula Tutorial (aka Elemental Analysis)

Below is a tutorial on doing Empirical Formula Problems. Grab a pencil and your notes and work the problem as the tutorial explains it to you. If you follow these simple 5 guidelines shown below, understanding Empirical Formulas should be no problem.

Example

(a) Given the following mass percent composition, determine the empirical formula.
49.5%C, 5.2%H, 28.8%N, 16.5%O

• Assume 100g sample [RULE 1]:

        49.5g C, 5.2g H, 28.8g N, 16.5g O

• Divide each mass number by the molar mass [RULE 2]:

            49.5 g C
        --------------- = 4.121 mol C
        12.011 g/mol C 

             5.2 g H
        --------------- = 5.159 mol H
        1.008 g/mol H

            28.8 g N   
        --------------- = 2.056 mol N
        14.007 g/mol N
 
            16.5 g O
        --------------- = 1.031 mol O
        15.999 g/mol O

• Divide each molar amount by smallest amount [RULE 3]:

        4.121 mol C      4 mol C
        -----------  =  ---------
        1.031 mol O       mol O

        5.159 mol H      5 mol H
        -----------  =  ---------
        1.031 mol O       mol O

        2.056 mol N      2 mol N
        -----------  =  ---------
        1.031 mol O       mol O

        1.031 mol O      1 mol O
        -----------  =  ---------
        1.031 mol O       mol O

Remember, empirical formula is C₄H₅N₂O

     Molar Mass  = (4x12.011)+(5x1.008)+(2x14.007)+(1x15.999)
                 =  97.097 g/mol

The empirical formula is NOT the chemical formula in this case. But the chemical formula
must be some integer factor of the empirical formula (thus, the molar mass of the chemical
formula must be some integer factor of the empirical formula).

(Factor) x Molar Mass Empirical Formula = Molar Mass Chemical Formula
(Factor) x 97.097 g/mol = 194.2 g/mol
(Factor) =2

Therefore, the chemical formula is C₈H₁₀N₄O₂

***NOTE: 2C₄H₅N₂O DOES NOT EQUAL C₈H₁₀N₄O₂***

Comments?

Last Revised : Tuesday, December 9, 1997

This page was created by Hank Bell as a Chemistry 1201/2 class project.