Percent Composition by Mass Calculator
Calculate the percent composition by mass of each element in a compound.
Enter up to 6 elements and get molar mass, percentages, and empirical formula hint.
Percent Composition
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:
- Write the molecular formula
- Find the molar mass of each element present (from periodic table)
- Multiply by the number of atoms of that element
- Divide by total molar mass of compound
- 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