Calculating Empirical Formula of an Organic Compound

Did you know that you can use the empirical formula on organic compounds?
Now what is an organic compound?
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compound whose molecules contain carbon.

Ex. When a 3.79 grams of an organic compound is burned, 6.61 grams of CO2 and 3.59 grams of H2O are produced.  What is the empirical formula of this compound?


Molar mass of CO2: 12 + 16 + 16 = 44 g/mol
Mole C = 6.61g x 1 mole CO2 x      1 mole of C   =  0.15023 mole of C
                              44.0 g CO2      1 mole of CO2

Molar mass of H20: 1.0 + 1.0 + 16.0 = 18.0 g/mol
Mole H = 3.59 g x 1 mole of H2O  x     2 moles of H  =  0.39889 mole of H
                              18.0 g H2O          1 mole of H20

Check the mass of C and H
Mass of C = 0.15023 x   12.0 g C = 1.80 g C
                                      1 mole C

Mass of H = 0.39889 x   1.0 g H   =  0.39889 g H
                                     1 mole H
Since the masses of C and H do not add up to 3.79, the rest of the mass must be from O.

Mass of O = 3.79 - 1.80 - 0.39889 = 1.59111

Mole O = 1.59 x 1 mole O  =  0.0994 g O
                           16.0 g O

Now we divide by the smallest molar amount. That is 0.0994

Carbon: 0.15023/0.0994 = 1.5 x 2 = 3
Hydrogen: 0.39889/0.0994 = 4.0 x 2 = 8
Oxygen: 0.0994/0.0994 = 1 x 2 = 2

Empirical formula: C2H8O2

This video will explain determining the empirical formula of an organic compound:

Empirical and Molecular Fomula

What is the empirical formula?
It gives the lowest term ratio of atoms (or moles) in a formula.

Ex. C4H10 (butane) --> molecular fomula
      Reduce the subscripts to lowest terms to get the empirical formula
      C2H5 --> empirical formula

Ex. Consider that we have 10.87 grams of Fe and 4.66 g of O.  What is the empirical formula?
First, we must convert the grams to moles.
10.87g x 1mole  =  0.195 moles of Fe
             55.8g

4.66g x 1 mole  =  0.291 moles of O
             16g

Now, we divide the mole amount by the smallest mole which is 0.195

Fe: 0.195  =  1
      0.195

O: 0.291  =  1.49 = 1.5
     0.195
Next, we must get those two numbers to the smallest whole number
Fe 1 x 2 = 2
O 1.5 x 2 = 3
The empirical formula is: Fe2O3

Ex. A compound contains 31.9% of K, 28.9% of Cl, and 39.2% of O.  What is the empirical formula?
*assume we have 100g*

31.9 x 1 mol  =  0.816 moles of K
           39.1g

28.9 x 1 mol  =  0.814 moles of Cl
           35.5g

39.2 x 1 mol  =  2.45 moles of O
           16g

lowest mole: 0.814
0.816  =  1.00
0.814

0.814  =  1
0.814

  2.45  =  3
0.814

Empirical formula: KClO3

What is the molecular formula?
It is a multiple of the empirical formula and shows the actual numbers of atoms that combine to form a molecule.

To calculate the multiple, we use this formula:
n =      molar mass of the compound    
      molar mass of the empirical formula


Ex. A molecule has an empirical formula of C2H5 and the molar mass is 58 g/mol. What is the molecular formula?
Molar mass is C2H5 = 58 g/mol
Molar mass of the empirical formula (total molar mass of all elements in the compound):
C2 = 12 x 2 = 24
H5 = 1 x 5 = 5
C2H5 = 29 g/mol

58 g/mol  =  2
29 g/mol

2 x C2H5 = C4H10

Video about the empirical formula:



Video about the molecular formula: