3.11 - Empirical & Molecular Formulae
What an empirical formula is
- An empirical formula is the simplest whole-number ratio of the atoms or ions in a compound.
- It's also called the simplest formula.
Giant structures
For a giant structure, the empirical formula is the only formula it has, because there are no separate molecules:
- Silicon dioxide is SiO2 because silicon and oxygen are in a 1:2 ratio.
- Sodium chloride is NaCl because sodium and chloride ions are in a 1:1 ratio.
Molecular substances
- For substances made of separate molecules:
- Molecular formula - The actual number of atoms of each element in one molecule.
- Empirical formula - The simplest whole-number ratio of atoms in the compound.
- Where the subscripts share no common factor, the two formulas are identical (e.g. water is H2O, methane is CH4).
- Where the subscripts share a common factor, divide each subscript by the largest common factor to get the empirical formula:
| Substance | Molecular formula | Empirical formula |
|---|---|---|
| Ethane | C2H6 | CH3 |
| Hydrogen peroxide | H2O2 | HO |
| Benzene | C6H6 | CH |
| Glucose | C6H12O6 | CH2O |
Worked example - Empirical formula of ethane
Ethane has the molecular formula C2H6. Deduce its empirical formula.
Step 1 - Read the number of atoms of each element
- Carbon: 2 atoms
- Hydrogen: 6 atoms
Step 2 - Find the largest whole number that divides both counts
Both 2 and 6 divide exactly by 2.
Step 3 - Divide each count by that number and write as subscripts
- Carbon: 2 ÷ 2 = 1
- Hydrogen: 6 ÷ 2 = 3
A subscript of 1 isn't written, so the empirical formula of ethane is CH3.
TermDefinition
Empirical formulaA formula that shows the simplest whole-number ratio of the atoms or ions of each element in a compound.
Molecular formulaA formula that shows the actual number of atoms of each element in one molecule.