Percent Composition by Mass Calculator

How Percent Composition Is Calculated

Percent composition tells you what fraction of a compound’s total mass comes from each element. It’s fundamental to stoichiometry, empirical formula determination, and quality control in chemistry.

Formula: % Element = (Mass of Element in 1 mol / Molar Mass of Compound) × 100

Steps:

1. Write the molecular formula
2. Find the molar mass of each element present (from periodic table)
3. Multiply by the number of atoms of that element
4. Divide by total molar mass of compound
5. Multiply by 100

Worked Example, Water (H₂O):

• Molar mass of H = 1.008 g/mol × 2 = 2.016 g/mol
• Molar mass of O = 15.999 g/mol × 1 = 15.999 g/mol
• Total molar mass = 18.015 g/mol
• % H = (2.016 / 18.015) × 100 = 11.19%
• % O = (15.999 / 18.015) × 100 = 88.81%
• Check: 11.19 + 88.81 = 100% ✓

Worked Example — Glucose (C₆H₁₂O₆), MW = 180.16 g/mol:

• C: (6 × 12.011) / 180.16 × 100 = 40.00%
• H: (12 × 1.008) / 180.16 × 100 = 6.71%
• O: (6 × 15.999) / 180.16 × 100 = 53.29%

Applications:

• Verifying compound purity in pharmaceutical manufacturing
• Determining empirical formulas from combustion analysis
• Nutrition labels (macronutrient percentages by mass)
• Mining: ore grade expressed as percent metal composition