Percent Composition Calculator

Percent composition tells you what fraction of a compound mass comes from each element. It is used to determine empirical formulas and verify compound identity in analytical chemistry.

📊 Stoichiometry📐 % = (n×Mᵢ/Mᵀ)×100⚗️ Analysis

Compound formula (e.g. H2O, CaCO3, C6H12O6)

Please enter valid values.

Formula & Reference

Variable	Symbol	Formula	Units
Percent Element	% X	%X = (n×M_X/M_total) × 100	%
Check	—	All percentages must sum to 100%	—
From Experiment	% X	(mass X in sample / total sample mass) × 100	%

Step-by-Step Examples

Example 1

Water H₂O

Find percent composition of water.

M = 2(1.008) + 16.00 = 18.02 g/mol
H: 2.016/18.02 × 100 = 11.19%
O: 16.00/18.02 × 100 = 88.81%

✓ H: 11.19%, O: 88.81% (sum = 100%)

Example 2

Glucose C₆H₁₂O₆

Find percent composition of glucose.

M = 180.16 g/mol
C: 72.066/180.16×100 = 40.00%
H: 12.096/180.16×100 = 6.71%
O: 95.994/180.16×100 = 53.29%

✓ C: 40.00%, H: 6.71%, O: 53.29%

Example 3

Iron(III) Oxide Fe₂O₃

Find % Fe in rust.

M(Fe₂O₃) = 2(55.845)+3(15.999) = 159.69 g/mol
Fe: 111.69/159.69×100 = 69.94%
O: 47.997/159.69×100 = 30.06%

✓ Fe: 69.94%, O: 30.06%

Real-World Applications

🧪

Compound Identification

Measured elemental percentages are compared to theoretical values to identify unknown compounds.

📊

Empirical Formula

Percent composition data is used to calculate empirical and molecular formulas.

🏭

Quality Control

Industries verify product purity by checking percent composition against theoretical values.

🌍

Environmental Analysis

Percent composition of pollutants determines source identification and remediation strategies.

Common Mistakes to Avoid

⚠️

Not summing to 100%

All elemental percentages must add to 100%. If not, recheck your calculation or rounding.

⚠️

Using mass instead of moles

Percent composition uses mass fractions: % = (n×Mᵢ / Mᵀ) × 100, not mole fractions.

⚠️

Confusing percent composition with mole percent

Mole percent = (moles of element / total moles) × 100. Mass percent uses mass fractions. They give different numbers.

Frequently Asked Questions

How do I use percent composition to find empirical formula? ▾

Assume 100 g sample: % becomes grams. Convert each to moles, divide by smallest, round to whole numbers.

What if percentages are given from experiment? ▾

Divide each element mass by its atomic mass to get moles. Find the simplest whole-number ratio.

Can percent composition identify an unknown compound? ▾

It narrows it down significantly. Combined with molar mass, you can uniquely identify the compound.

What is the difference between empirical and molecular formula? ▾

Empirical is simplest whole-number ratio (CH₂O for glucose). Molecular is actual (C₆H₁₂O₆ for glucose).

Why must composition sum to 100%? ▾

Mass is conserved. Every atom in the compound contributes to the total mass. The fractions must account for all atoms.

Related Chemistry Calculators

Formula Explorer connections

Interpretation: This relationship converts chemical amount, mass, composition or balanced-equation ratios into a reaction quantity. Assumption: Use a balanced reaction, consistent units and the correct molar mass. Purity, side reactions and limiting reagents can change experimental results.

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