Percent Composition

What is Percent Composition?

Percent composition (also called mass percent) tells you the percentage of each element's mass relative to the total molar mass of the compound. It is calculated for each element as:

Mass % = (number of atoms × atomic mass of element / molar mass of compound) × 100

Steps to Calculate

1. Parse the chemical formula to find each element and how many atoms are present
2. Look up the atomic mass of each element from the periodic table
3. Calculate each element's total mass contribution by multiplying atoms × atomic mass
4. Sum all contributions to get the molar mass of the compound
5. Divide each contribution by the molar mass and multiply by 100

Example: Water (H₂O)

• H: 2 atoms × 1.008 g/mol = 2.016 g/mol
• O: 1 atom × 15.999 g/mol = 15.999 g/mol
• Molar mass = 18.015 g/mol
• H: (2.016 / 18.015) × 100 = 11.19%
• O: (15.999 / 18.015) × 100 = 88.81%

Supported Formula Notation

• Simple formulas: H2O, NaCl, CO2
• Parenthetical groups: Ca(OH)2, Al2(SO4)3
• Hydrates: CuSO4.5H2O or CuSO4·5H2O
• Nested parentheses: Ca3(PO4)2

Applications

• Determining nutritional content of supplements and food additives
• Verifying experimental results in analytical chemistry
• Identifying unknown compounds from combustion analysis data
• Quality control in pharmaceutical and industrial chemistry

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