How to find an empirical formula
- Assume 100 g, so each percentage becomes grams.
- Convert to moles: divide each mass by the element's atomic mass.
- Divide by the smallest number of moles.
- Make them whole. If a ratio is close to .5, multiply all by 2; close to .33 or .67, by 3; close to .25 or .75, by 4.
For the molecular formula, divide the molar mass by the empirical formula mass. Glucose has empirical formula CH₂O (30 g/mol) and molar mass 180, so n = 6 and the molecular formula is C₆H₁₂O₆.
Don't round too early. A ratio of 1.33 is not 1; it means multiply everything by 3. Keep two decimal places until the last step.
Check the result with the molar mass calculator.
Questions students ask
What is the difference between empirical and molecular formula?
The empirical formula is the simplest whole-number ratio of atoms (CH₂O). The molecular formula is the actual number in one molecule (C₆H₁₂O₆), always a whole-number multiple of the empirical formula.
A compound has 40% C, 6.7% H and 53.3% O. What is its empirical formula?
CH₂O. The moles are 3.33, 6.7 and 3.33, which give the ratio 1 : 2 : 1.
What if the percentages don't add up to 100?
If only oxygen is missing, the rest is usually oxygen: subtract the others from 100. Otherwise, it's fine to use the masses as given.