Percent Composition Calculator
Type a chemical formula to see what percentage of its mass each element makes up, with a bar for each share and the grams of each element in a sample if you give its mass. Or switch to a mixture and enter the weighed masses of its components. Free, no sign-up.
Case matters: Co is cobalt, CO is carbon monoxide. Hydrates use · or *.
Fill this in to get the mass of each element in your sample.
Results
159.687 g/mol
Molar mass
69.94%
Iron
30.06%
Oxygen
Mass percent of each element
- Iron (Fe)69.94%
- Oxygen (O)30.06%
| Element | Mass in one mole (g) | Mass % |
|---|---|---|
| Iron (Fe) | 111.69 | 69.94% |
| Oxygen (O) | 47.997 | 30.06% |
The working
Fe: 111.69 ÷ 159.687 × 100 = 69.94%
O: 47.997 ÷ 159.687 × 100 = 30.06%
Rounded percentages can add up to 99.99% or 100.01%; the unrounded shares add up to exactly 100%.
How to Use the Percent Composition Calculator
- 1. Choose what you are analysing. Pick the elements in a compound to work from a formula, or the components of a mixture or solution to work from masses you weighed.
- 2. Enter the formula or the masses. A formula can include brackets and hydrates (Ca3(PO4)2, CuSO4·5H2O). For a mixture, name each component and enter its mass; a solution is two components, the solute and the solvent.
- 3. Add a sample mass if you want grams. In formula mode, an optional sample mass turns the percentages into the mass of each element in that sample.
- 4. Pick the rounding and read the working. Choose one to four decimal places. Every percentage is shown as the part's mass divided by the total, times 100.
The Percent Composition Formula
Mass percent
mass % of A = (mass of A / total mass) × 100
In a compound
mass % of element = (n × atomic weight of element / molar mass of compound) × 100
For a compound, take one mole as the sample: the mass of the element in one mole is its atomic weight times the number of its atoms in the formula, and the total is the compound's molar mass. The result does not depend on how much of the compound you have, which is the law of definite proportions — pure Fe2O3 is the same percentage iron whether you have a gram or a kilogram. The molar masses come from the IUPAC abridged atomic weights used by the Molar Mass Calculator.
Worked Examples
Iron in iron(III) oxide, Fe2O3. The calculator's default formula. The molar mass is 2 × 55.845 + 3 × 15.999 = 111.69 + 47.997 = 159.687 g/mol.
- Iron: 111.69 ÷ 159.687 × 100 = 69.94%
- Oxygen: 47.997 ÷ 159.687 × 100 = 30.06%
- In a 50.0 g sample that is 34.97 g of iron and 15.03 g of oxygen.
A hydrate. Copper(II) sulfate pentahydrate, CuSO4·5H2O, has a molar mass of 249.677 g/mol, of which the five waters supply 90.075 g. Water is therefore 36.08% of the crystal and anhydrous CuSO4 is 63.92%. When a lab heats a hydrate and the mass falls by about 36%, the sample was probably the pentahydrate. Switch the formula in the calculator to see the share of each part.
A mixture. A student separates a sample of 6.20 g sand, 3.80 g salt and 2.00 g iron filings (the calculator's default mixture). The total is 12.00 g, so sand is 6.20 ÷ 12.00 × 100 = 51.67%, salt 31.67% and iron 16.67%. The three rounded values add to 100.01%, so a small rounding drift is normal.
Mass percent is not mole percent or volume percent
Percent composition here is always by mass. A gas mixture quoted as 21% oxygen is usually 21% by volume (and by moles), which is a different number from its mass percent. For concentrations of solutions, use the Percent Solution Calculator, which separates w/w, w/v and v/v.
Common Mistakes
- Using the wrong denominator. Divide by the molar mass of the whole compound, not by the mass of the other element.
- Forgetting the subscript. Oxygen in Fe2O3 contributes 3 × 15.999, not 15.999.
- Ignoring the water of hydration. A formula with ·nH2O has the water in its total.
- Expecting rounded percentages to add to 100. Report them as rounded and check the total against 100 within the last digit.
- Rounding too soon. Keep the full atomic weights until the final percentage; the Significant Figures Calculator shows how many digits to report.
Frequently Asked Questions
- How do you calculate percent composition by mass?
- Find the molar mass of the compound, then for each element divide the total mass of that element in the formula by the molar mass and multiply by 100. For water, 2 × 1.008 ÷ 18.015 × 100 = 11.19% hydrogen and 15.999 ÷ 18.015 × 100 = 88.81% oxygen.
- What is the difference between percent composition and empirical formula?
- They are inverse problems. Percent composition starts from a formula and gives the mass percentages; an empirical formula starts from the percentages (or masses) and gives the simplest whole-number atom ratio. The Empirical Formula Calculator handles the second.
- How do I find the mass of an element in a sample?
- Multiply the sample mass by the element's mass fraction, which is its percentage divided by 100. In 50.0 g of Fe2O3, the iron is 50.0 × 0.6994 = 34.97 g. Enter the sample mass in the calculator to see this for every element at once.
- Why don't my percentages add up to exactly 100%?
- Each percentage is rounded separately, so the displayed values can total 99.99% or 100.01%. The unrounded shares always sum to 100%. If your total is off by more than the last digit, a subscript or atomic weight was probably entered wrongly.
- What is mass percent for a solution?
- It is the mass of the solute divided by the mass of the whole solution (solute plus solvent), times 100. Enter the solute and the solvent as two components in mixture mode: 5.0 g of salt in 95.0 g of water is 5.0 ÷ 100.0 × 100 = 5.0% by mass.
- Does the nitrogen content of a fertilizer come from percent composition?
- Yes. Ammonium nitrate, NH4NO3, has a molar mass of 80.043 g/mol and 2 × 14.007 = 28.014 g of nitrogen per mole, which is 35.00% nitrogen by mass. Fertilizer grades are quoted on the same percent-by-mass basis.