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StoichiometryPercent compositionContent level: Core 20 min

Percent Composition

What you'll be able to do: Find the mass percent of any element in a compound, and use percent composition to test whether a sample matches a formula.

Best after: The Mole and Molar Mass

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Introduction

A chemical formula tells you how many atoms of each element are present. Percent composition turns that counting information into something you can weigh: what fraction of a sample's mass comes from each element. It is the bridge between a formula on paper and a balance reading in the laboratory.

These are recommended, not required. You can start this lesson at any time.

Learning objectives

  • Calculate the molar mass of a compound from its formula.
  • Calculate the mass percent of each element in a compound.
  • Use percent composition to decide whether an unknown sample is consistent with a proposed formula.
  • Convert between mass percent and the mass of an element in a weighed sample.

Lesson

What percent composition actually measures

Percent composition answers a mass question, not a counting question. In one mole of water the two hydrogen atoms contribute only 2.016 g out of 18.015 g, so hydrogen is 11.19 % of the mass even though two out of every three atoms are hydrogen. Whenever an answer feels surprising, ask whether you were thinking about atom counts when the question asked about mass.

Percent by mass and percent by atoms are different numbers. Read the question carefully.

The three-step routine

1. Write the formula and expand it atom by atom. 2. Multiply each subscript by that element's atomic mass and add the results to get the molar mass. 3. Divide each element's total mass by the molar mass and multiply by 100. Keeping the arithmetic in this order means the molar mass is computed once and reused, which removes most rounding errors.

% element = (n x atomic mass of element) / M(compound) × 100 %

Parentheses and hydrates

A subscript outside parentheses multiplies every atom inside it, so Ca(NO₃)₂ contains 1 Ca, 2 N and 6 O. A dot in a hydrate formula means the water is part of the formula unit and its mass counts: CuSO₄ . 5 H₂O has a molar mass of 249.68 g/mol, not 159.61 g/mol.

Ca(NO₃)₂, CuSO₄ . 5 H₂O

Expand the formula into a short atom list before you touch the calculator.

Using percent composition in the laboratory

Percent composition is how a mass measurement becomes a chemical claim. If a 5.00 g sample of a white solid is found to contain 2.06 g of sodium, that is 41.2 % Na, which matches NaCl (39.34 %) poorly and matches Na₂CO₃ (43.38 %) better than NaHCO₃ (27.37 %). Reasoning like this is the first half of empirical formula work in the next lesson.

Next: Empirical and Molecular Formulas

Checking your answer

The percentages of all elements in a compound must sum to 100 % within rounding. If they sum to 97 % or 104 %, an atomic mass was mistyped or a subscript was missed. A second check: the element with the largest total mass contribution must have the largest percentage, which is not always the element with the most atoms.

Key ideas

Definition
Percent composition

The mass percent of each element in a compound, calculated from one mole of that compound.

Rule
Mass, not count

Percent composition weighs atoms. A light element with many atoms can still be a small percentage of the mass.

Rule
Percentages sum to 100

Every element's percentage added together must give 100 % within rounding. Use this as a built-in check.

Assumption
Pure substance

The formula describes the whole sample. A mixture has no single percent composition, each component must be handled separately.

Equation
Mass percent of an element

% X = (nX x AX) / M x 100 %

  • nX = number of atoms of element X in one formula unit (exact)
  • AX = atomic mass of X in g/mol
  • M = molar mass of the whole compound in g/mol
  • use when = you know the formula and want a mass fraction
  • limits = the sample must be the pure compound; impurities shift every percentage
Equation
Mass of an element in a sample

mX = (% X / 100) x msample

  • mX = mass of element X in the sample, in g
  • msample = total mass of the weighed sample, in g
  • use when = a question asks how much of one element a weighed sample contains
  • limits = assumes the sample is entirely the named compound

Worked examples

Worked example 1

Calculate the percent composition of Ca(NO₃)₂.

Try it first: Expand the formula into an atom list before you calculate anything.

    0 of 4 steps revealed.

    Worked example 2

    A fertiliser is pure ammonium sulfate, (NH₄)₂SO₄. How many grams of nitrogen are in a 50.0 g sample?

    Try it first: Decide whether you need a percentage or a direct mass ratio, both routes work.

      0 of 3 steps revealed.

      Common mistakes

      Counting only some of the atoms inside parentheses, for example 3 O in Ca(NO₃)₂.

      Why it's wrong: The outside subscript multiplies every atom inside the parentheses, so there are 6 O and 2 N.

      Check instead: Write the expanded atom list on its own line before adding masses.

      Reporting the fraction of atoms instead of the fraction of mass.

      Why it's wrong: Hydrogen is 2 of the 3 atoms in water but only 11.19 % of its mass, because atoms have very different masses.

      Check instead: Ask whether the question says percent by mass; if so, every term must be a mass.

      Leaving the water of crystallisation out of a hydrate's molar mass.

      Why it's wrong: The waters are part of the formula unit, so CuSO₄ . 5 H₂O has M = 249.68 g/mol, not 159.61 g/mol.

      Check instead: Treat everything after the dot as extra atoms in the formula.

      Accepting percentages that do not add to about 100 %.

      Why it's wrong: A sum well away from 100 % means an atomic mass or subscript is wrong, the individual answers cannot all be right.

      Check instead: Add your percentages as the last step, every time.

      Letting periodic-table masses set the significant figures of the final answer.

      Why it's wrong: Atomic masses are reference data known to more digits than the measured sample mass.

      Check instead: Count significant figures from the measured quantities in the problem.

      Practice this skill

      No practice questions are available for this topic yet. You can still practice the whole unit.

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      What you should now know

      Percent composition is the mass of one element in one mole of a compound divided by the molar mass of that compound, expressed as a percentage. Compute the molar mass first, keep the element masses as mass per mole, and check that all the percentages add to 100 %.

      • Percent composition is a mass fraction calculated from one mole of the compound.
      • Expand parentheses and hydrate waters before computing the molar mass.
      • All element percentages must sum to 100 % within rounding.
      • Percent composition is independent of sample size, so it scales directly to any mass.
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      Sources and further reading

      • Chemistry 2e, Section 3.1: Formula Mass and the Mole Concept
        Paul Flowers, Klaus Theopold, Richard Langley, William R. Robinson, et al. · OpenStax, Rice University · Chapter 3.1 · pp. Section 3.1 (web edition)
        View source

        Adapted from "Chemistry 2e" by OpenStax, Rice University, licensed CC BY 4.0. Changes were made. License