BackElement Mass % in Compound (Percent Composition) Calculator
Enter a chemical formula and an element symbol. The calculator shows the molar mass breakdown and the percent by mass of that element in one formula unit.
Background
This tool parses chemical formulas—including parentheses, nesting, and hydrates (dot notation)— to count atoms, compute each element’s mass contribution, and return the element’s mass percent: % element = (mass of element in formula unit ÷ total molar mass) × 100%.
How to use this percent composition calculator
1) Type a valid chemical formula (case sensitive for element symbols).
2) Enter a one- or two-letter element symbol (e.g., O, Na, Cl).
3) Click Calculate Percent Composition to see the element’s mass and percent, plus a full breakdown table.
Key Points
- Parentheses and nesting are supported: Al2(SO4)3.
- Hydrates (dot notation) work: CuSO4·5±á2O or CuSO4·5±á2O.
- % element = (n(element) × AW(element) ÷ total molar mass) × 100%.
Formula & Equation Used
Let M be the molar mass of the compound. If an element E appears n(E) times, then its mass contribution is n(E) × AW(E) and %E = 100 × n(E) × AW(E) / M.
- AW(E): atomic weight of element E (g/mol)
- M: total molar mass (g/mol) of the formula unit
Example Problems & Step-by-Step Solutions
Example 1
Find the percent by mass of oxygen in glucose, C6H12O6.
- Molar mass = 6(12.011) + 12(1.008) + 6(15.999) = 180.156 g/mol
- Mass of O = 6(15.999) = 95.994 g/mol
- %O = 100 × 95.994 / 180.156 ≈ 53.28%
Example 2
Find the percent by mass of nitrogen in (NH4)2SO4.
- Molar mass = 2(14.007) + 8(1.008) + 32.06 + 4(15.999) = 132.134 g/mol
- Mass of N = 2(14.007) = 28.014 g/mol
- %N = 100 × 28.014 / 132.134 ≈ 21.20%
Frequently Asked Questions
Q: Does it handle hydrates like CuSO4·5±á2O?
Yes—use a middle dot (·) or a period (.). The parser multiplies each part correctly.
Q: Which atomic weights are used?
Standard IUPAC-style table values. For strict grading, match your class data table.
Q: How many decimals should I report?
Choose a sensible number of decimals (default 2) and follow your course’s sig fig guidance.
