Empirical and Molecular Formula
Simple Explanation
An empirical formula shows the simplest whole-number ratio of atoms in a compound. A molecular formula shows the actual number of atoms in one molecule — it can be the same as the empirical formula, or a whole-number multiple of it.
Why Do We Need It?
Knowing the ratio of atoms lets chemists figure out exactly what a compound is made of, even before they know its full molecular structure.
See It
A ball-and-stick diagram of a water molecule, showing one oxygen atom bonded to two hydrogen atoms
Worked Example
Find an empirical formula from a molecular formula
A compound has the molecular formula C₆H₁₂O₆ (glucose). What is its empirical formula?
Why Does This Work?
Dividing every subscript by their highest common factor keeps the same ratio between elements while reducing it to its simplest whole-number form — exactly what 'empirical formula' means.
Real-Life Example
Identifying an unknown compound
A chemist burns an unknown sample and measures the masses of CO₂ and water produced.
Working backwards from those masses to a mole ratio, and then to an empirical formula, is a standard real technique for identifying an unknown organic compound.
Practice
A compound has the molecular formula C₂H₄. What is its empirical formula?
MediumCommon mistake
Assuming the empirical and molecular formula are always different — sometimes, like with water (H₂O), they're already identical because the ratio is already in its simplest form.
Quick Review
- Empirical formula = simplest whole-number ratio of atoms.
- Molecular formula = actual number of atoms in one molecule.
- Divide the molecular formula's subscripts by their highest common factor to get the empirical formula.