Percent Composition by Mass Calculator

How Percent Composition Is Calculated

Percent composition tells you what fraction of a compound’s total mass comes from each element. It’s fundamental to stoichiometry, empirical formula determination, and quality control in chemistry.

Formula: % Element = (Mass of Element in 1 mol / Molar Mass of Compound) × 100

Steps:

1. Write the molecular formula
2. Find the molar mass of each element present (from periodic table)
3. Multiply by the number of atoms of that element
4. Divide by total molar mass of compound
5. Multiply by 100

Worked Example, Water (H₂O):

• Molar mass of H = 1.008 g/mol × 2 = 2.016 g/mol
• Molar mass of O = 15.999 g/mol × 1 = 15.999 g/mol
• Total molar mass = 18.015 g/mol
• % H = (2.016 / 18.015) × 100 = 11.19%
• % O = (15.999 / 18.015) × 100 = 88.81%
• Check: 11.19 + 88.81 = 100% ✓

Worked Example, Glucose (C₆H₁₂O₆), MW = 180.16 g/mol:

• C: (6 × 12.011) / 180.16 × 100 = 40.00%
• H: (12 × 1.008) / 180.16 × 100 = 6.71%
• O: (6 × 15.999) / 180.16 × 100 = 53.28%

Those three add to 99.99, not 100.00, and that is only rounding. Carried to four places they are 40.0021, 6.7142 and 53.2837, which sum exactly. If your own figures miss by a whole percent or more, the problem is the formula rather than the arithmetic.

Applications:

• Verifying compound purity in pharmaceutical manufacturing
• Determining empirical formulas from combustion analysis
• Nutrition labels (macronutrient percentages by mass)
• Mining: ore grade expressed as percent metal composition

Entering formulas with parentheses

This calculator takes element-and-count pairs rather than a formula string, so anything in brackets has to be multiplied out first. Calcium hydroxide, Ca(OH)₂, is entered as Ca 1, O 2, H 2, giving a molar mass of 74.09 g/mol. Aluminum sulfate, Al₂(SO₄)₃, becomes Al 2, S 3, O 12.

Watch the oxygen count in that second one. The subscript 4 inside the bracket multiplies by the 3 outside it, which catches people out far more often than the metal does.

Why measured composition drifts from the theoretical figure

If you run a combustion analysis and the percentages come back a few points off, the usual culprit is water rather than a bad measurement. Many salts crystallize with water locked into the lattice, and that water counts toward the mass. Copper sulfate pentahydrate, CuSO₄·5H₂O, is 36% water by mass, so its copper percentage is 25.5% against the 39.8% you would calculate for the anhydrous salt.

To handle a hydrate here, enter the water’s hydrogen and oxygen along with everything else: CuSO₄·5H₂O is Cu 1, S 1, O 9, H 10.