Example

If the subscripts in the formula have a common divisor, it is usually a molecular formula. Generally the empirical formula is multiplied by this common divisor to get the molecular formula.
Example:
Empirical formula of acetic acid is CH2O.
Molecular formula is CH3 COOH = C2H4O2
Numericals based on empirical formula
Example:1
An oxide of iron contains 72.41% of iron. Calculate the empirical formula for the oxide of iron [Fe = 56; O=16].
Solution

Empirical formula = Fe3O4.
Steps for calculation
- Calculate the percentage by weight of each element.
- Find out relative number of atoms by dividing percentage of weight by atomic weight.
- Choose the simplest ratio and the smallest, divide all the ratios by it.
- If whole numbers are not obtained, then multiply it by a smallest integer to make it whole.
Example: 2
The percentage composition of a compound is 71.8% antimony (Sb) and 28.2% sulphur. What is the empirical formula of this compound?
Solution:
In 100g of the compound, masses of elements are as follows:
In 100 grams of compound, 71.8 g are antimony and 28.2% sulphur.
Determination of molecular formula from empirical formula
Molecular formula is the chemical formula, which represents the actual numbers of atoms of each element present in a compound.Example: 1
Calculate the molecular formula of a compound with vapor density of 30 having 40% carbon; 6.67% of hydrogen and the rest is oxygen.
Empirical formula = C1H2O1
Empirical formula weight = 12 x 1 + 2 x 1 + 1 x 16= 12 + 2 + 16
= 30 gMolecular weight = 2 x vapor density
= 2 x 30= 60
Molecular weight = n x empirical weight60 = n x 30
Molecular formula = n x empirical formula
= 2 x CH2O = C2H4O2Steps
- Calculate empirical formula.
- Use vapor density if given.
- If molecular weight given, calculate 'n' using this formula.
- Molecular formula = n x empirical formula
Example: 2
A compound has molecular formula C5H10. What is its empirical formula?
Solution:
Ratio of C atoms to H atoms is 5 : 10 = 1:2.
Empirical formula is C1H2.