Generated on: June 17, 2026 at 05:24 PM

Percent Composition Calculator

Inputs

Atomic Mass of Element16 g/mol
Number of Atoms1
Molar Mass of Compound18.015 g/mol

Generated with OneCalc · https://onecalc.app/en/chemistry/percent-composition-calculator

Chemistry

Percent Composition Calculator

Find the mass percent of an element in a compound from the number of atoms, atomic mass, and molar mass of the compound. Mass % = (n × Aᵣ ÷ M) × 100.

Inputs

g/mol
g/mol

Results

Enter a value to see results.

%

Details

g/mol

Embed this calculator

Preview

Paste this code into your page to show the calculator.

Share this calculation

Anyone who opens this link sees your values already filled in.

Understanding percent composition

Percent composition is the mass fraction of each element in a compound, expressed as a percentage. It answers the question: of every 100 grams of this compound, how many grams come from a particular element? The value depends only on the compound's formula — not on how much of the compound you have.

The percent composition formula

For a single element in a compound:

w=n×ArM×100%w = \frac{n \times A_r}{M} \times 100\%
SymbolMeaning
wwMass percent of the element (%)
nnNumber of atoms of the element in one formula unit
ArA_rStandard atomic weight of the element (g/mol)
MMMolar mass of the compound (g/mol)

The numerator n×Arn \times A_r is the contribution of that element to the compound's molar mass. Dividing by MM gives the mass fraction; multiplying by 100 converts it to a percentage.

All elements in a compound sum to 100%:

∑iwi=100%\sum_i w_i = 100\%

Finding the molar mass

Before using the formula you need the compound's molar mass. Add the atomic masses of every atom in the molecular formula, multiplied by how many times each appears. For water (H₂O):

M(H2O)=2×1.008+1×15.999=18.015 g/molM(\text{H}_2\text{O}) = 2 \times 1.008 + 1 \times 15.999 = 18.015\ \text{g/mol}

Standard atomic weights are found on the periodic table.

Worked example — oxygen in water

Water has the formula H₂O. One oxygen atom (Aᵣ = 16.00 g/mol) sits in a molecule with molar mass 18.015 g/mol.

me=n×Ar=1×16.00=16.00 g/molw=16.0018.015×100%≈88.81%\begin{aligned} m_e &= n \times A_r = 1 \times 16.00 = 16.00\ \text{g/mol} \\[6pt] w &= \frac{16.00}{18.015} \times 100\% \approx 88.81\% \end{aligned}

Hydrogen accounts for the rest:

w(H)=2×1.00818.015×100%≈11.19%w(\text{H}) = \frac{2 \times 1.008}{18.015} \times 100\% \approx 11.19\%

The two percentages sum to 100%, confirming the calculation.

Percent composition vs. empirical formula

Percent composition and empirical formulas are closely related:

ConceptWhat it gives you
Percent compositionMass fraction of each element (a ratio)
Empirical formulaSimplest whole-number atom ratio
Molecular formulaActual atom count per molecule

You can derive an empirical formula from percent composition by treating the percentages as grams (assume 100 g of compound), dividing each element's mass by its atomic weight to get moles, and then finding the smallest whole-number ratio of moles.

Practical uses

Percent composition is used in combustion analysis, where a known mass of compound is burned and the combustion products (CO₂ and H₂O) are weighed to determine the carbon and hydrogen content. It also appears in nutritional labeling, alloy specifications, and quality control in manufacturing, wherever knowing the elemental breakdown by mass is required.

Frequently Asked Questions (FAQ)

What is the percent composition formula?

The mass percent of an element in a compound is:

mass % = (n × Aᵣ ÷ M) × 100

where n is the number of atoms of the element in one formula unit, Aᵣ is the standard atomic weight of the element in g/mol, and M is the molar mass of the compound in g/mol. The result tells you what fraction of the compound's mass comes from that element.

How do I find the molar mass of a compound?

Add up the atomic masses of every atom in the molecular formula, weighted by how many times each element appears. For water (H₂O): two hydrogen atoms at 1.008 g/mol each plus one oxygen atom at 15.999 g/mol gives 2 × 1.008 + 15.999 = 18.015 g/mol. Standard atomic weights are listed on the periodic table and in reference tables such as IUPAC's atomic weights.

What is the difference between percent composition and an empirical formula?

Percent composition tells you the mass fraction of each element in a compound — for example, water is 88.8% oxygen and 11.2% hydrogen by mass. An empirical formula gives the simplest whole-number ratio of atoms — for water that is H₂O, which is already the molecular formula. You can use percent composition data to derive an empirical formula by converting mass percents to moles and then finding the smallest integer ratio.

What is the percent composition of oxygen in water?

Water (H₂O) has a molar mass of 18.015 g/mol. Oxygen contributes one atom with an atomic mass of 16.00 g/mol:

mass % O = (1 × 16.00 ÷ 18.015) × 100 ≈ 88.81%

Hydrogen accounts for the remaining two atoms: (2 × 1.008 ÷ 18.015) × 100 ≈ 11.19%. The two percentages sum to 100%, as expected.

Recommended Next

Mole Calculator

Convert between mass, moles, and molar mass with n = m ÷ M. Find moles from grams, grams from moles, or the molar mass — plus the number of particles.

 Learn more
Molarity Calculator

Calculate molarity (mol/L), moles, or solution volume using M = n ÷ V. Solve for any one of the three quantities given the other two.

 Learn more
Mass Percent Calculator

Calculate mass percent (weight/weight %) of a solution from solute and solution mass, or solve for the solute or solution needed. Mass % = (solute ÷ solution) × 100.

 Learn more

200+ calculators · 10 languages · 100% free

Was this calculator helpful?

No

How can we improve this calculator?
Powered by OneCalc ↗