How to Use This Empirical Formula Calculator
- Percent or mass: type each element’s symbol and its percent by mass or its mass in grams. Add the compound’s molar mass if you also want the molecular formula.
- Combustion analysis: type the mass of the sample burned and the masses of CO2 and H2O it produced.
The result shows the moles of each element, the ratios after dividing by the smallest, any multiplier needed, and the formula mass.
How to Find an Empirical Formula From Percent Composition
The empirical formula is the lowest whole-number ratio of atoms in a compound, which is how the AP course defines it in Topic 1.3, Elemental Composition of Pure Substances. To find it:
- Assume a 100 g sample, so each percent becomes grams.
- Divide each mass by the element’s molar mass to get moles.
- Divide every mole value by the smallest one.
- If any ratio is not close to a whole number, multiply all of them by the same small whole number.
Example: ascorbic acid is 40.92% C, 4.58% H, and 54.50% O. In 100 g: C 40.92 ÷ 12.01 = 3.407 mol, H 4.58 ÷ 1.008 = 4.544 mol, O 54.50 ÷ 16.00 = 3.406 mol. Dividing by 3.406 gives 1.000 : 1.334 : 1.000. Because 1.334 is about 1⅓, multiply by 3: 3 : 4 : 3, so the empirical formula is C3H4O3.
When to Multiply the Mole Ratios
| Ratio ends in about | Fraction | Multiply every ratio by |
|---|---|---|
| .50 | ½ | 2 |
| .33 or .67 | ⅓ or ⅔ | 3 |
| .25 or .75 | ¼ or ¾ | 4 |
| .20, .40, .60, or .80 | fifths | 5 |
A ratio within about 0.1 of a whole number can be rounded, because measured percentages carry small errors. Anything further away should be multiplied, not rounded: 1.5 rounded to 2 turns Fe2O3 into FeO2. The calculator tries multipliers from 1 to 8 and uses the smallest one that brings every ratio within 0.1 of a whole number.
How to Find the Molecular Formula From the Molar Mass
The molecular formula is a whole-number multiple of the empirical formula. Divide the molar mass of the compound by the empirical formula mass, then multiply every subscript by the result. For ascorbic acid, C3H4O3 has a formula mass of 3(12.01) + 4(1.008) + 3(16.00) = 88.06 g/mol. With a molar mass of 176.12 g/mol, n = 176.12 ÷ 88.06 = 2.000, so the molecular formula is C6H8O6. Any formula mass can be checked with the molar mass calculator, and the method is worked by hand for glucose in AP Chemistry empirical and molecular formula.
How Combustion Analysis Gives an Empirical Formula
When a compound of carbon and hydrogen burns completely, all of its carbon ends up in CO2 and all of its hydrogen in H2O. Combustion analysis uses that to find the composition. Burning 0.255 g of isopropyl alcohol gives 0.561 g CO2 and 0.306 g H2O:
- Carbon: 0.561 ÷ 44.01 = 0.01275 mol C, or 0.1531 g.
- Hydrogen: 0.306 ÷ 18.02 × 2 = 0.03396 mol H, or 0.03423 g.
- Oxygen by difference: 0.255 − 0.1531 − 0.03423 = 0.0677 g, or 0.004230 mol O.
Dividing by 0.004230 gives 3.01 : 8.03 : 1, so the empirical formula is C3H8O. Its formula mass, 60.09 g/mol, equals the molar mass of isopropyl alcohol, so here the molecular formula is the same. The mass relationships are the same stoichiometry used for any reaction.
Formula questions are a small part of Unit 1. To see how your practice scores across all nine units translate into a 1–5 estimate, use the AP Chem Score Calculator.
Related Resources
- AP Chem Score Calculator
- All AP Chemistry Calculators
- AP Chemistry Empirical & Molecular Formula
- Molar Mass Calculator
- AP Chemistry Mole Conversions
- Percent Yield Calculator
- Unit 1 Review: Atomic Structure and Properties
Frequently Asked Questions
How do you find an empirical formula from percent composition?
Assume a 100 g sample so each percent becomes grams, divide each mass by the element’s molar mass to get moles, then divide every mole value by the smallest one. If the ratios are not all whole numbers, multiply them all by the same small number (2, 3, 4, or 5) until they are.
What is the difference between an empirical formula and a molecular formula?
The empirical formula is the lowest whole-number ratio of atoms, such as CH2O. The molecular formula is the actual number of atoms in one molecule, such as C6H12O6, and is always a whole-number multiple of the empirical formula. Ionic compounds are written with empirical formulas.
How do you get the molecular formula from the empirical formula?
Divide the molar mass of the compound by the empirical formula mass to get a whole number n, then multiply every subscript by n. For CH2O (30.03 g/mol) and a molar mass of 180.16 g/mol, n = 6, giving C6H12O6.
What do you do when a mole ratio is 1.5 or 1.33?
Do not round it. A ratio ending in .5 means multiply every ratio by 2, one ending in .33 or .67 means multiply by 3, and one ending in .25 or .75 means multiply by 4. Rounding 1.5 to 2 gives the wrong formula.
How does combustion analysis give an empirical formula?
Burning a compound of carbon, hydrogen, and oxygen turns all of its carbon into CO2 and all of its hydrogen into H2O. The mass of CO2 gives the moles of C, the mass of H2O gives the moles of H (two per water), and oxygen is the sample mass minus the carbon and hydrogen. Those moles are then reduced to a whole-number ratio.
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