Guides
How to Calculate Mass Percent — Formula & Examples
Mass percent (percent by mass) is the fraction one component contributes to a total mass, written as a percentage. It shows up in two places: the share of solute in a solution, and the share of an element in a compound. The formula is the same shape — the trick is knowing what goes on top and bottom.
Two formulas
Solute mass percent = solute mass ÷ solution mass × 100%, where solution mass = solute + solvent.
Element mass percent = (relative atomic mass × number of atoms) ÷ compound molar mass × 100%.
Steps
- Decide which mass percent Is the question about a solute in a solution, or an element in a compound? Pick the matching formula.
- Find the numerator The solute mass, or the element's total mass (relative atomic mass × atoms).
- Find the denominator The total solution mass, or the compound's molar mass.
- Divide and ×100% Numerator ÷ denominator × 100% gives the percentage.
Worked examples
| Problem | Calculation | Result |
|---|---|---|
| 20 g salt dissolved in 80 g water | 20 ÷ (20+80) × 100% | 20% |
| %H in H₂O | (2×1.008) ÷ 18.02 × 100% | ≈ 11.2% |
| %Ca in CaCO₃ | 40.08 ÷ 100.09 × 100% | ≈ 40.0% |
| %N in NH₄NO₃ | (2×14.01) ÷ 80.04 × 100% | ≈ 35.0% |
Common mistakes
- The denominator is the solution mass (solute + solvent), not just the solvent.
- For element mass percent, the molar mass must count every atom (see how to calculate molar mass).
- Mass percent differs from molarity: one is a mass ratio, the other is mol/L — convert via molarity.
Type any formula into the molar mass calculator to read off each element's mass percent.