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General Chemistry I · Stoichiometry

Percent Composition

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Key takeaway
  6. Study tools
  7. Sources & references

In 30 seconds

Percent composition is the mass percentage of each element in a compound — the mass of one element divided by the total mass of the compound, multiplied by 100. It is an intensive property: the percentage is identical whether you analyze 1 gram or 1 kilogram of the substance, because the formula fixes the mass ratio of its elements. Percent composition is calculated from a chemical formula, and it is the starting data used to work backward and find an empirical formula.

Why this matters

Percent composition is how a laboratory's elemental analyzer reports an unknown — it gives back "40.0% C, 6.7% H, 53.3% O," and the chemist uses those numbers to deduce the formula. It is also used in quality control (checking a fertilizer's nitrogen content), nutrition (macronutrient percentages), and mining (ore grade as percent metal).

The college version

Key Ideas

What the percentage means

  • Percent composition answers "what fraction of this compound's mass is element X?"
  • It is a mass ratio, not a particle ratio — 18.02 g of water contains 2.02 g of hydrogen.

Why the percentages are fixed

  • The chemical formula fixes the atom ratio; each atom type has a fixed mass, so the mass ratio (and percentage) is constant for a pure compound.
  • This constancy is the basis of the law of definite proportions.

Formula → percent, and percent → formula

  • Formula → percent composition: divide each element's total mass by the molar mass.
  • Percent composition → empirical formula: the reverse process (see the empirical formula note).

Equations and Variables

  • Percent by mass of element X:

%X = (mass of X in one mole of compound / molar mass of compound) × 100%

  • Equivalently: %X = [(n × atomic mass of X) / M(compound)] × 100%, where n = number of X atoms per formula unit.

How It Works (Problem-Solving Method)

  1. Compute the molar mass of the compound.
  2. Find each element's total mass (number of atoms × atomic mass).
  3. Divide each element's total mass by the molar mass.
  4. Multiply by 100% for the percentage.
  5. Check that all percentages sum to ≈ 100% (allowing rounding).

Worked Example

Find the percent composition of water, H₂O.

  • Molar mass of H₂O = 2(1.008) + 16.00 = 18.016 g/mol.
  • Mass of H = 2 × 1.008 = 2.016 g/mol.
  • Mass of O = 1 × 16.00 = 16.00 g/mol.

%H = (2.016 / 18.016) × 100% = 11.19%

%O = (16.00 / 18.016) × 100% = 88.81%

  • Check: 11.19% + 88.81% = 100.00%. Water is 11.19% hydrogen and 88.81% oxygen by mass.

Common Confusions

  • "H₂O is 2/3 hydrogen." — By count of atoms, yes (2 of 3 atoms); by mass it is only 11.2% hydrogen, because H is so light.
  • "Percent composition changes with sample size." — No; 5 g and 500 g of pure water are both 88.81% oxygen.
  • "The percentages don't have to add to 100%." — In a pure compound they must (within rounding); a sum far from 100% signals an error or an impure sample.
  • "Percent composition is measured directly from a formula." — You can calculate it from a formula; measuring it (combustion analysis) is how you go the other direction.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine a sandwich that always has the same recipe: two slices of bread and one slice of cheese. If bread weighs 1 unit a slice and cheese weighs 16 units, the whole sandwich weighs 18. The cheese is 16 of those 18 units — about 89% cheese by weight, even though it's only one of three slices. Water is like that: hydrogen is the light "bread" and oxygen the heavy "cheese," so oxygen is 89% of water's weight even though water has twice as many hydrogen atoms. The catch: the analogy uses slices of different weight, which is exactly the point — percentage is about mass, not about how many pieces there are.

Key takeaways

  • %X = (mass of X in the compound / molar mass) × 100%.
  • Percent composition is independent of sample size (intensive property).
  • Percentages must sum to ~100% for all elements.
  • It is computed from a formula and used in reverse to get empirical formulas.
  • Mass percent is by mass, not by number of atoms.
  • Percent composition = (element mass / molar mass) × 100%.
  • Fixed by the formula; independent of sample size.
  • Sums to 100% for all elements.
  • Connects formulas to empirical-formula determination.

Keep learning

Ready to build on this? Continue to the next lesson.

Practice General Chemistry I

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Study tools & related lessonsYou’ll learn to · Related

You’ll learn to

  • Define percent composition and its formula.
  • Calculate the mass percent of each element in a compound from its formula.
  • Use percent composition as a bridge to empirical formulas.
  • Verify that the mass percents of a compound sum to 100%.

Sources & references

  1. OpenStax, "3.2 Determining Empirical and Molecular Formulas." *Chemistry 2e*.
  2. Chemistry LibreTexts, "10.10: Percent Composition."
  3. Chemistry LibreTexts, "10: The Mole."

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