Percent Composition Calculator

The Makeup of Matter: A Guide to Percent Composition

Percent composition is a fundamental concept in chemistry that describes the percentage by mass of each element present in a chemical compound. It essentially tells you what proportion of a compound's total mass comes from each of its constituent elements. For example, when we say water (H₂O) is about 11% hydrogen and 89% oxygen by mass, we are describing its percent composition. This information is crucial for identifying unknown compounds, verifying the purity of a substance, and understanding the stoichiometry of chemical reactions. It is the bridge between a chemical formula and the real-world mass relationships of the elements within it.

This calculator is a straightforward tool designed to automate the calculation of percent composition. By simply entering the chemical formula of a compound, the tool will parse the formula, calculate the total molar mass of the compound, and then determine the mass percentage contributed by each individual element. This saves you the time of looking up atomic masses and performing the multi-step calculation manually. It's an excellent resource for chemistry students learning how to perform these calculations and for scientists who need a quick and accurate result in the lab.

How to Calculate Percent Composition

The calculation is a three-step process:

1. Find the Molar Mass: First, calculate the total molar mass of the entire compound by summing the atomic masses of all the atoms in its formula.
2. Find the Total Mass of Each Element: For each element in the compound, multiply its atomic mass by the number of atoms of that element in the formula (its subscript).
3. Calculate the Percentage: For each element, divide its total mass from step 2 by the total molar mass of the compound from step 1, and then multiply the result by 100 to get the percentage.
   Percent Composition = (Total Mass of Element / Molar Mass of Compound) × 100%

Example: Percent Composition of Water (H₂O)

• Molar Mass of H ≈ 1.008 g/mol; Molar Mass of O ≈ 16.00 g/mol.
• Step 1: Total Molar Mass of H₂O = (2 × 1.008) + 16.00 = 18.016 g/mol.
• Step 2: Total mass of H = 2 × 1.008 = 2.016 g/mol. Total mass of O = 1 × 16.00 = 16.00 g/mol.
• Step 3:
  % Hydrogen = (2.016 / 18.016) × 100% ≈ 11.19%
  % Oxygen = (16.00 / 18.016) × 100% ≈ 88.81%

This calculator automates this entire process for any chemical formula you enter.