Empirical and Molecular formula

The empirical formula of a compound is the simplest whole number ratio of atoms of each element in the compound. It is determined using data from experiments.
The molecular formula shows the actual number of atoms of each element present in a compound.

 Example Molecular formula Empirical formula C4H8 CH2 C6H6 CH CH4 CH4

To find empirical table, follow the steps below and present it in a table.

1         Write the element symbols
2         Write the masses
3         Write the values of Ar
4         Divide masses by Ar to find mole
5         Divide by the smallest number to find mole ratio
6         Write the formula

Example:
Find the empirical formula of a compound that contains 3.2g of sulfur and 3.2g of oxygen.

 Element S O Mass (g) 3.2 3.2 Ar 32 16 Mole 3.2 ÷ 32 = 0.1 3.2 ÷ 16 = 0.2 Mole ratio 0.1 ÷ 0.1 = 1 0.2 ÷ 0.1 = 2

The empirical formula has one atom of sulfur and two atoms of oxygen.
Empirical formula is SO2

You can use this method to find the degree of hydration in salt crystals.

Example:
210g of magnesium salt crystals, MgSO4.nH2O was heated strongly in a crucible to remove water. The final mass of anhydrous solid left was 120g. What is the formula of this salt crystal?

 Compound MgSO4 H2O Mass (g) 120 210-120=90 Mr 120 18 Mole 120 ÷ 120 = 1 90 ÷ 18 = 5 Mole ratio 1 5

Formula is MgSO4.5H2O

To find molecular formula, you need to find the Mr of empirical formula and molecular formula. Divide Mr of molecular formula by Mr of empirical formula below to find n:

Mr of molecular formula    =   n
Mr of empirical formula

 Example Molecular formula Empirical formula n C4H8 CH2 4 C6H6 CH 6 CH4 CH4 1

Molecular for formula = (empirical formula)n

Example:
The empirical formula of glucose is CH2O. Its relative molecular mass is 180. Find the molecular formula.

n = Mr of molecular formula    =          180         =   6
Mr of empirical formula          ( 12 + 2+ 16)

molecular formula = (CH2O)6 = C6H12O6