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