Percent tells you mass. Formulas count atoms. Convert to moles to switch.
An empirical formula is the simplest whole-number ratio of atoms. A molecular formula is the real count in one molecule, which is always a whole-number multiple of the empirical formula. To get from one to the other you need the molar mass.
Four compounds share the exact same percent composition (40.0% C, 6.7% H, 53.3% O), so they share one empirical formula, CH₂O:
30.03 g/mol · ×1
60.05 g/mol · ×2
90.08 g/mol · ×3
180.16 g/mol · ×6
That's why the molar mass is required to get a molecular formula. Percent alone can't tell lactic acid from glucose.
Step-by-step solver
Percent composition from a formula
Find the formulas
This is how unknowns get identified.
Combustion analysis burns a sample and weighs the CO₂ and H₂O that come out. All the carbon ends up in CO₂ and all the hydrogen in H₂O, so those masses give you grams of C and H, and oxygen is whatever mass is left. Add a mass spectrometer reading for molar mass and you have the molecular formula. It's the same two-step logic as the solver above, done with real instruments.
Common mistakes
- Rounding 1.5 to 2 (or 1.33 to 1). If a ratio ends near .5, multiply everything by 2. Near .33 or .67, multiply by 3. Near .25 or .75, multiply by 4.
- Dividing percent by percent. Convert each to moles first, then divide by the smallest mole value.
- Using the molar mass of the compound in step 2. Each element gets its own atomic mass.
- Forgetting oxygen when the problem only gives C and H. If the percents don't add to 100, the rest is usually O.