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: