Empirical and Molecular Formulas

Empirical Formula: The ratio of the number of elements in a compound expressed in lowest form
Ionic Compounds are usually an example of empirical formulas

Example: NaCl

               MgO

Molecular Formula: The actual amount of the number of elements in a specific compound
Some covalent compound's molecular formula is not the same as their empirical formula
Example: C₄H₁₀  (The empirical form would be C₂H₅)               H₂O₂   (The empirical form would be HO)To convert from and Empirical to Molecular or Molecular to Empirical, we need to find the whole number to multiply it by.

Converting Between Empirical and Molecular Formulas:

The three formulas are:

Molecular Formula = Empirical × Whole Number (N)

Mass of 1 mole = Empirical mass in grams × Whole Number

Molecular Formula Mass = Empirical × Whole Number

If the empirical formula of a compound is C₂H₅and the molar mass is 58g/mol, what is the molecular formula for this compound?
First, we find the mass the C2H5 (Empirical Formula)
C = 12.0x2 = 24.0g
H = 1.0x 5 = 5.0g
24.0+5.0 = 29.0g


Then, divide the molar mass by the mass of the empirical formula to calculate the whole number (N) used to separate between the emiprical and the molecular formulas.


58g/mol ÷29.0 g = 2 

Find the empirical formula of a compound that contains 7.2grams of Carbon, 1.2grams of Hydrogen, and 9.6grams of Oxygen.

Step 1: Convert the grams of elements into moles

C:  7.2g C×1 mol C/12.0g C = 0.6 mol

H:  1.2g H×1 mol H/1.0g H = 1.2 mol

O:  9.6g O×1 mol O/16.0g O = 0.6 mol

Step 2: Divide the moles by the smallest amount of mole to get the empirical formula

C:  0.6mol C/0.6mol C = 1

H:  1.2mol H/0.6mol C = 2

O:  0.6mol O/0.6 molC = 1

Therefore, the empirical formula for this compound would be: CH₂O

For further examples, here is a video to show you the some conversions between empirical and molecular formulas.