Empirical formulas
The empirical formula is the simplest whole-number ratio of atoms in a compound. The molecular formula is the actual number of atoms in a molecule. Glucose has the molecular formula C6H12O6 and the empirical formula CH2O.
To find an empirical formula:
- Start with the mass of each element. If given percentages, assume a 100 g sample so each percent becomes grams.
- Convert each mass to moles.
- Divide every mole value by the smallest one.
- If a result is not close to a whole number, multiply all of them by a small number (2, 3 and so on) until they are.
Worked example: 40.0% C, 6.7% H, 53.3% O
In 100 g there are 40.0 g of C, 6.7 g of H and 53.3 g of O.
| Element | Moles | Divided by smallest |
|---|---|---|
| C | 40.0 ÷ 12.011 = 3.33 | 1.00 |
| H | 6.7 ÷ 1.008 = 6.65 | 2.00 |
| O | 53.3 ÷ 15.999 = 3.33 | 1.00 |
Empirical formula: CH2O
To get a molecular formula, divide the compound’s molar mass by the empirical formula mass, then multiply every subscript by the result. If CH2O (30.03 g/mol) has a molar mass of 180.16 g/mol, the multiplier is 6 and the molecular formula is C6H12O6.
Common mistake
Rounding too early. A ratio of 1.5 is not 2. Multiply everything by 2 to get whole numbers: 1 to 1.5 becomes 2 to 3.