Chapter 6 · Chemical Composition
6.7Mass Percent Composition from a Chemical Formula
11 min · two checks
Predict
What is the mass percent of O in H₂O?
The idea
- Compute mass percent from a formula.
- Use one mole of the compound as the convenient sample.
Pretend you have one mole. The “sample mass” is the molar mass. The mass of each element is its subscript times its atomic mass. Divide and multiply by 100%. For sulfuric acid, H₂SO₄, the molar mass is 98.08 g/mol. Sulfur’s share is 32.07 / 98.08 × 100% = 32.70%. You never needed a real beaker.
Check that the percents sum to 100. If they do not, an atom was dropped, usually inside parentheses. Mass percent is also a conversion factor in disguise: 32.70% S means 32.70 g S per 100 g H₂SO₄. That is often faster than the full mole map when the question is only “grams of element in grams of compound.”
Keep these
- Use 1 mol of compound as the sample.
- mass % element = (sub × atomic mass / molar mass) × 100%.
- Percents must total 100 within rounding.
Worked path
Find the mass percent of oxygen in CaCO₃. Ca 40.08, C 12.01, O 16.00.
Observe
Three O atoms. Molar mass = 40.08 + 12.01 + 48.00 = 100.09 g/mol.