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 by mass tells you what fraction of a compound’s mass comes from each element.
Formula:
% element = (n × M_element / M_compound) × 100
Where:
- n = number of atoms of that element in one formula unit
- M_element = atomic molar mass of the element (g/mol)
- M_compound = molar mass of the entire compound (g/mol)
Step-by-step:
- Determine the molecular formula
- Calculate molar mass of the compound
- For each element: divide (n × M_element) by M_compound and multiply by 100
Example: Glucose (C₆H₁₂O₆), M = 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%
- Total = 100% ✓
Common atomic masses (g/mol): H=1.008, He=4.003, C=12.011, N=14.007, O=15.999, F=18.998, Na=22.990, Mg=24.305, Al=26.982, Si=28.086, P=30.974, S=32.06, Cl=35.45, K=39.10, Ca=40.08, Fe=55.85, Cu=63.55, Br=79.90, Ag=107.87, I=126.90, Ba=137.33, Pb=207.2
Application — empirical formula: Percent composition is the starting point for determining the empirical formula. Divide each percentage by atomic mass to get mole ratios, then find the simplest whole-number ratio.