How to use the empirical formula calculator
- Choose whether the amounts are percent by mass or grams.
- Enter each element symbol and its amount, adding a row for each element.
- Optionally enter the molar mass to find the molecular formula, or a multiplier of your own.
- Read the mole ratios, the whole-number formula and, if given, the molecular formula.
Formula
Moles of each element = mass / atomic weight (percentages are treated as grams in 100 g)
Divide every amount of moles by the smallest to get the ratios
Multiply the ratios by the smallest whole number (1 to 10) that makes all of them whole, within 0.1
Molecular formula = empirical formula x (molar mass / empirical formula mass), when that is a whole number
Fractions like 1.5, 1.33 and 1.25 need multipliers of 2, 3 and 4. The result is written in Hill order: C, then H, then the rest alphabetically.
From percent composition to formula
A compound that is 40.00% carbon, 6.71% hydrogen and 53.29% oxygen has mole ratios of 1 : 2 : 1, so the empirical formula is CH2O with a mass of 30.026 g/mol. With a molar mass of 180.16 the multiple is 6, and the molecular formula is C6H12O6, which is glucose. Benzene, 92.26% C and 7.74% H, has the empirical formula CH and, at 78.11 g/mol, the molecular formula C6H6.
Iron oxide with 69.94 g of iron and 30.06 g of oxygen has a ratio of 1 : 1.5, so the multiplier is 2 and the formula is Fe2O3. Aspirin, 60.00% C, 4.48% H and 35.52% O, has ratios 2.25 : 2 : 1, so the multiplier is 4 and the empirical formula C9H8O4 is also its molecular formula at 180.16 g/mol.
Empirical and molecular formulas
The empirical formula is the simplest whole-number ratio of atoms in a compound, found from lab measurements of its composition. The molecular formula shows the actual number of atoms in one molecule, and is a whole-number multiple of the empirical formula. Glucose and formaldehyde share the empirical formula CH2O but differ in molar mass. The molar mass has to come from another measurement.
To convert between mass and moles, use the moles to grams calculator. For reactions, the limiting reagent calculator shows which reactant runs out first.
When the ratios are not whole
Real data has rounding and small measurement error, so ratios like 1.98 or 2.03 should be read as 2. The page accepts a ratio within 0.1 of a whole number. If no multiplier up to 10 makes every ratio whole, it reports the ratios and asks you to check the data or choose a multiplier yourself. Percentages that do not add up to about 100 are rejected as an incomplete analysis.
The atomic weights come from an abridged table of standard values with 3 to 5 significant digits, so the empirical mass is accurate to about 0.01. For converting concentrations after you know the formula, see the concentration unit converter.
Assumptions and limits
- The composition covers all elements in the compound, and percentages add up to 100 within 2.
- Atomic weights are abridged standard values; elements outside the table are rejected with a message.
- Ratios must be within 0.1 of a whole number after the multiplier.
- A molecular formula is given only when the molar mass is close to a whole multiple of the empirical mass.
Frequently asked questions
What is an empirical formula?
The simplest whole-number ratio of the atoms in a compound, such as CH2O for glucose.
How do I get the molecular formula?
Divide the molar mass by the empirical formula mass. If the result is a whole number n, multiply every subscript by n.
Can I enter grams instead of percent?
Yes. Choose Grams as the unit and enter the mass of each element from your analysis.
What if the ratio is 1 to 1.5?
Multiply both parts by 2. The page finds this multiplier for you, up to 10.