Empirical and molecular formulas
Empirical and molecular formulas
- The empirical formula is the simplest whole-number ratio of atoms.
- The molecular formula is the actual number of atoms in a molecule.
- Some crystals also trap water — water of crystallisation.
Practice
Ethane has empirical formula CH₃ and molecular formula C₂H₆. This shows:
Empirical = simplest whole-number ratio; molecular = the real number of atoms per molecule.
Finding the empirical formula
From masses (or % by mass):
- divide each element's mass by its $A_r$ → moles.
- divide all the mole values by the smallest.
- round to whole numbers — that ratio is the empirical formula.
Example: ethane has empirical formula $\text{CH}_3$ but molecular formula $\text{C}_2\text{H}_6$.
Practice
The first step in finding an empirical formula from masses is to:
Convert masses to moles (mass ÷ Ar), then divide by the smallest and round to whole numbers.
From empirical to molecular
- You also need $M_r$.
- Find how many times the empirical formula mass fits into $M_r$.
- Multiply the empirical formula by that number.
Practice
To get the molecular formula from the empirical formula, you also need:
Divide Mr by the empirical formula mass, then multiply the empirical formula by that whole number.
Hydrated and anhydrous
- Water of crystallisation is water held inside a crystal.
- A solid that contains it is hydrated; with the water removed it is anhydrous.
- E.g. hydrated copper(II) sulfate $\text{CuSO}_4 \cdot 5\text{H}_2\text{O}$ → heat → anhydrous $\text{CuSO}_4$.
Practice
An anhydrous salt is one that:
Hydrated salts hold water of crystallisation; heating drives it off to give the anhydrous form.
You've got it
Key idea
- empirical = simplest ratio; molecular = actual atoms (ethane $\text{CH}_3$ vs $\text{C}_2\text{H}_6$)
- empirical: mass ÷ $A_r$ → divide by smallest → round
- molecular: multiply empirical by ($M_r$ ÷ empirical-formula mass)
- hydrated holds water of crystallisation; anhydrous has it removed