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Composition & Empirical Formula Calculator

Get mass percent composition from a formula, or find empirical and molecular formulas from percent data.

Composition & empirical formula

Mass percent from a formula, or empirical/molecular formula from % data

Molar mass 180.16 g/mol

ElementCountMass %
C640.001%
H126.7138%
O653.285%

What is the Composition & Empirical Formula Calculator?

Percent composition states how much of a compound’s mass comes from each element. It follows directly from the chemical formula and atomic masses: divide each element’s contribution by the molar mass and multiply by 100%. Combustion analysis and elemental analysis report percentages that you then convert into an empirical formula—the simplest whole-number mole ratio of atoms.

If an independent molecular mass is known (from mass spectrometry or gas-density data), multiply the empirical formula by an integer so the molar mass matches. The Composition & Empirical Formula Calculator handles both directions: formula → mass percents, and percent/mass data → empirical (and optional molecular) formula.

  • % element = (mass of element in 1 mol) / (molar mass) × 100%
  • Empirical formula from moles of each element, scaled to smallest integers
  • Molecular formula = (empirical) × n, where n ≈ M_molecular / M_empirical

Mathematical / chemical formulas

Core relations:

%X = (n_X × A_X / M) × 100%

moles of X = (mass % of X) / A_X   (using 100 g sample)
Divide each mole amount by the smallest → relative indices
Clear fractions to the smallest integers → empirical formula

Step-by-step example: Empirical formula from 40.0% C, 6.7% H, 53.3% O

An organic compound is 40.0% C, 6.7% H, and 53.3% O by mass. Its molar mass is about 180 g/mol. Find the empirical and molecular formulas.

  1. In 100 g: 40.0 g C, 6.7 g H, 53.3 g O.
  2. Moles: C 3.331, H 6.647, O 3.331 → ratios ≈ 1 : 2 : 1.
  3. Empirical formula CH₂O (M ≈ 30.03 g/mol).
  4. n = 180 / 30 ≈ 6 → molecular formula C₆H₁₂O₆.

Use Empirical from % / mass with C 40, H 6.7, O 53.3 and molecular mass 180.

Frequently asked questions

Do percents have to sum to exactly 100?

Experimental values often sum to 99–101% because of rounding. The calculator treats values near 100% as mass percents; if amounts look like grams with a sum far from 100, it treats them as relative masses.

Why might indices like 1.5 appear?

Mole ratios are not always integers before scaling. Multiplying by 2 clears a 1.5 ratio (for example CH₃O → C₂H₆O₂).

References & further reading

Standards bodies, university open courseware, and peer-reviewed references that align with the methods used on this page.

Keep learning with more calculators and study guides on Online Science Tools.

Practice problems & worked examples

Practice alongside the composition calculator above. Each problem includes a full worked solution so you can check your reasoning step by step.

Practice problem 1

Percent oxygen in water

What is the mass percent of oxygen in H₂O?

Show solution

Worked solution

  1. M(H₂O) ≈ 18.015 g/mol; O contributes ≈ 15.999 g/mol.
  2. %O ≈ (15.999/18.015)×100% ≈ 88.81%.

Answer: %O ≈ 88.8%

Practice problem 2

Empirical formula

A compound is 40.0% C, 6.7% H, 53.3% O. Find the empirical formula.

Show solution

Worked solution

  1. Moles in 100 g: C 3.33, H 6.65, O 3.33 → ratio 1:2:1.
  2. Empirical formula CH₂O.

Answer: CH₂O

Practice problem 3

Molecular formula

Empirical formula CH₂O has M ≈ 30 g/mol. Molecular mass ≈ 180 g/mol. Molecular formula?

Show solution

Worked solution

  1. n = 180/30 = 6.
  2. Molecular formula C₆H₁₂O₆.

Answer: C₆H₁₂O₆

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