Try:
C6H12O6 — molar mass 180.156 g/mol
| Element | Atoms | Mass % | Grams in sample | Atom % |
|---|---|---|---|---|
| O Oxygen | 6 | 53.28% | 53.28 g | 25.00% |
| C Carbon | 6 | 40.00% | 40 g | 25.00% |
| H Hydrogen | 12 | 6.71% | 6.714 g | 50.00% |
Mass % is each element's share of the compound's mass: (atoms × atomic mass) ÷ molar mass × 100. Atom % is its share of the atoms — the two differ because atoms have different masses. In water, hydrogen is two-thirds of the atoms but only about 11% of the mass.
How it works
The formula is parsed into atom counts (including groups in parentheses and water in hydrates), and each element's mass contribution — atom count × standard atomic weight — is divided by the total molar mass. With a sample mass, each percentage is simply applied to that mass.
Frequently asked questions
- How do you calculate percent composition?
- For each element, multiply its atom count by its atomic mass, divide by the molar mass of the whole compound, and multiply by 100. In water, hydrogen contributes 2 × 1.008 = 2.016 g/mol out of 18.015 g/mol, which is 11.19%.
- Do the percentages always add up to 100%?
- Yes — every gram of the compound belongs to one of its elements. If a set of lab percentages adds up to much less than 100%, an element (often oxygen) was not measured and is usually taken as the remainder.
- What is the difference between mass percent and atom percent?
- Mass percent is each element’s share of the mass; atom percent is its share of the atoms. Heavy atoms dominate by mass: in water, hydrogen is 66.7% of the atoms but only 11.2% of the mass.
- How is percent composition used?
- It is the starting point for finding an empirical formula from lab analysis, for checking a compound’s purity, and for working out how much of an element (for example, iron in a supplement or nitrogen in a fertilizer) a given mass contains.
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