Percent Composition Calculator

Percent Composition Explained

Percent composition tells you what fraction of a compound's total mass comes from each element — useful for everything from checking a compound's purity to reversing the calculation to find an unknown formula.

$$\%\text{ element} = \frac{\text{mass of element in formula}}{\text{molar mass of compound}} \times 100\%$$

Each element's contribution to the total mass (its atomic mass times how many times it appears) is divided by the compound's overall molar mass, then converted to a percentage.

Worked Example: Glucose

Using the calculator's default formula, C₆H₁₂O₆ (glucose) — with a molar mass of about 180.16 g/mol — carbon contributes 6 × 12.011 = 72.066 g/mol, which is 72.066/180.16 ≈ 40.0% of the total mass. Hydrogen contributes about 6.7%, and oxygen makes up the remaining 53.3%. Despite having the most individual atoms, hydrogen contributes the least to the mass, since it's by far the lightest element in the formula.

Percent Composition and Empirical Formulas

Percent composition is the reverse of a very practical lab technique: burn an unknown compound, measure how much of each combustion product forms, and back-calculate the percent composition of the original sample. From there, converting each percentage to a mole ratio (assuming a 100 g sample, so percent equals grams directly) reveals the empirical formula — the simplest whole-number ratio of elements, which is often the very first clue chemists have about an unidentified substance.

A Brief History of Compositional Analysis

Antoine Lavoisier's combustion experiments in the 1770s and 80s were among the first systematic attempts to determine what compounds are made of by mass, laying groundwork for the law of definite proportions. Joseph Proust formalized that law in 1799, establishing that a pure compound always has the same percent composition by mass no matter its source or how it was prepared — a principle so fundamental it's easy to take for granted today, but revolutionary at the time.

Common Percent Composition Mistakes

Forgetting to multiply an element's atomic mass by its subscript count before dividing is a common error — using just the atomic mass instead of the full contribution badly understates that element's percentage. Confusing percent composition (by mass) with percent by mole count is another frequent mix-up, since the two can differ substantially for elements with very different atomic masses. Rounding each percentage separately without checking that they sum to 100% is a third common oversight — a good sanity check after any percent composition calculation.

Percent Composition Terms You Should Know

Percent Composition — the mass percentage each element contributes to a compound's total mass.

Empirical Formula — the simplest whole-number ratio of elements in a compound, often derived from percent composition.

Law of Definite Proportions — the principle that a pure compound always has the same percent composition by mass.

Combustion Analysis — a lab technique for determining percent composition by burning a sample and measuring products.

Percentages shown are rounded for display; the underlying calculation uses full precision atomic masses.