Empirical formula calculator with the ratio table

An empirical formula is the simplest whole-number ratio of atoms in a compound. To find it, change each element's mass (or percentage) into moles by dividing by its atomic mass, divide every result by the smallest, and scale to whole numbers. A compound that is 40% carbon, 6.7% hydrogen and 53.3% oxygen has the empirical formula CH₂O. This empirical formula calculator shows that table in full and gives the molecular formula too when you add the relative molecular mass.

It opens on that compound. Open the full empirical formula calculator over this page to try your own.

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Empirical Formula Calculator

Find empirical and molecular formulae from masses or percentages

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How to use the empirical formula calculator

1. Choose Percentages or Masses.
2. For each element, pick it from the periodic table and type its percentage or mass in grams. Add or remove rows as needed.
3. Optionally type the Relative molecular mass, Mr to get the molecular formula.
4. Read the empirical formula, its mass and the molecular formula, with the ratio table: moles, divided by the smallest, and the whole-number ratio.
5. Ratios such as 1.5 or 1.33 are scaled up automatically. To practise, press Random problem in the Practice section.

How to calculate the empirical formula

1. Write down the mass of each element. If you have percentages, imagine 100 g, so 40% becomes 40 g.
2. Divide each mass by the element's relative atomic mass to get moles.
3. Divide every mole number by the smallest one.
4. If any result isn't close to a whole number, multiply them all by the same number to make them whole: × 2 for .5, × 3 for .33 or .67, × 4 for .25 or .75.
5. Write the whole numbers as subscripts.

For the molecular formula, divide Mr by the empirical formula's mass and multiply every subscript by the answer.

Worked examples you can check in the calculator

Example 1: from percentages, with the molecular formula

The empirical formula is CH₂O, with mass 30.03. If Mr = 180, then 180 ÷ 30.03 = 6, so the molecular formula is C₆H₁₂O₆ (glucose). These are the calculator's opening values.

Example 2: from masses, with a ratio of 1.5

11.2 g of iron combines with 4.8 g of oxygen. Moles: Fe = 11.2 ÷ 55.845 = 0.2006 and O = 4.8 ÷ 15.999 = 0.3000. Dividing by the smallest gives 1 : 1.5. Multiply both by 2: 2 : 3, so the formula is Fe₂O₃. Rounding 1.5 to 2 would give the wrong answer, FeO₂.

Example 3: a hydrocarbon

A hydrocarbon is 85.7% carbon and 14.3% hydrogen. Moles in 100 g: C = 7.135 and H = 14.19, a ratio of 1 : 1.99, so the empirical formula is CH₂ (mass 14.03). If Mr = 56, then 56 ÷ 14.03 = 4 and the molecular formula is C₄H₈.

Example 4: an oxide of phosphorus

An oxide is 43.6% phosphorus and 56.4% oxygen. Moles: P = 1.408 and O = 3.525, a ratio of 1 : 2.5. Doubling gives P₂O₅, with mass 141.94. With Mr = 284 the molecular formula is P₄O₁₀.

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Common mistakes

• Using masses as the ratio. Convert to moles first; atoms have different masses.
• Rounding 1.5 or 1.33 to the nearest whole number. Multiply everything up instead.
• Dividing by atomic number instead of atomic mass. Use the relative atomic mass.
• Rounding too early. Keep three or four significant figures until the final ratio.
• Forgetting oxygen given by difference. If only some percentages are given, the rest may be another element, often oxygen.

Read next

Chemistry · Moles and quantitiesMolar mass calculator: any formula, with percent compositionFind the molar mass of any chemical formula, including brackets, hydrates and ions, with each element's share by mass and worked examples to check.