Calculate the percentage by mass of each element within a chemical compound.
Stoichiometric Masses
g/mol
g/mol
Mass Contribution
Composition Results
Mass Percentage27.29%
The selected part represents 27.29% of the molecule's total mass of 44.01 g/mol.
Overview
Percent composition defines what portion of a compound's total mass is contributed by each individual element. It is calculated by taking the total mass of an element in one mole of a compound and dividing it by the compound's total molar mass. This analytical tool is essential for verifying chemical formulas and understanding the purity of a substance.
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Pro Tips
Total Element Mass: Remember to multiply the atomic weight by the number of atoms of that element in the formula (e.g., O₂ = 16.00 × 2).
Sum of Parts: The mass percentages of all elements in a compound must always sum to exactly 100%.
Purity Testing: If your experimental percent composition differs from the theoretical value, your sample is likely impure.
Empirical Formula: You can work backward from percent composition to find the empirical formula of an unknown substance.
Units: Grams, moles, and amu all work as long as you are consistent between the part and the whole.
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Fun Facts
"Caffeine (C₈H₁₀N₄O₂) is roughly 49.5% Carbon by mass, but only 5.2% Hydrogen."
"Water (H₂O) is about 88.8% Oxygen by mass, despite having twice as many Hydrogen atoms—Oxygen's atomic weight is 16 times higher!"
"The discovery of different 'isotopes' meant that the average percent composition of samples could vary slightly depending on their geological source."
"Early miners used percent composition to determine if an ore was 'rich' enough in metal to be worth smelting."
"Combustion analysis, a 200-year-old technique, is still the gold standard for measuring carbon and hydrogen percentage in organic chemistry."
Formula: % Element = (Total Element Mass / Compound Molar Mass) × 100
