Percent Composition Formula
Elemental makeup of a compound by mass percentage
Definition
% element = (mass of element in 1 mol compound / molar mass of compound) × 100
For formula calculations, use atomic masses and subscript counts to find element mass per mole of compound.
Steps
- Compute molar mass of the compound (sum atomic masses × counts).
- For each element, compute its mass contribution (atomic mass × count).
- Divide element mass by total molar mass and multiply by 100%.
Example: H2SO4
MM: 2(1.008) + 32.06 + 4(16.00) = 98.08 g/mol
H: 2(1.008) = 2.016 g → 2.016/98.08 × 100 ≈ 2.06%
S: 32.06 g → 32.06/98.08 × 100 ≈ 32.7%
O: 4(16.00) = 64.00 g → 64.00/98.08 × 100 ≈ 65.3%
Check: 2.06% + 32.7% + 65.3% ≈ 100%
Round at the end to avoid cumulative errors.
Common Pitfalls
Atomic mass precision
Use appropriate significant figures; keep extra decimals during calculation.
Forgetting subscripts
Multiply each element\'s atomic mass by its subscript count in the formula.
Rounding to 100%
Small rounding differences are normal; adjust one element slightly if necessary.
Hydrates and mixtures
Include waters of hydration or specify which compound mass basis is used.
FAQ
Can percent composition be by atoms instead?
Yes, but standard practice is by mass. Atom percent is used in materials contexts.
How to handle unknown formulas?
If only mass percentages are given, derive the empirical formula first, then compare to molar mass for molecular formula.
What about solutions?
Use mass percent formula (m_solute/m_solution × 100) for solutions rather than formula-based composition.