Percent composition by mass, step by step
How to turn a chemical formula into the mass percent of each element, why it matters, and where the numbers can trip you up.
From formula to molar mass
Percent composition starts with molar mass, the mass of one mole of a compound in grams. To find it, break the formula into its elements, look up each standard atomic weight, and multiply by how many atoms of that element appear. Adding those contributions gives the molar mass. For glucose, C6H12O6, that is six carbons at 12.011, twelve hydrogens at 1.008 and six oxygens at 15.999, which sum to 180.156 g/mol. Every percent that follows is measured against this single total.
Dividing the mass among the elements
Once the molar mass is known, the mass percent of an element is simply its share of that total. Take the element's combined mass, divide by the molar mass, and multiply by 100. In glucose the carbon contributes 72.066 g/mol, so 72.066 divided by 180.156 gives 40.00 percent. Hydrogen contributes 12.096 g/mol for 6.71 percent, and oxygen contributes 95.994 g/mol for 53.28 percent. Because oxygen atoms are heavy and numerous, they dominate the mass even though carbon and hydrogen supply more atoms.
Hydrates and grouped units
Real formulas often bundle atoms into groups or attach water molecules. Parentheses multiply everything inside them, so Ca(OH)2 means one calcium plus two oxygen and two hydrogen atoms. A centred dot marks a hydrate, and the number in front of the trailing water counts full molecules, so CuSO4.5H2O carries five waters of crystallisation into the mass total. Leaving out that water is a common mistake that quietly lowers the molar mass and shifts every percentage.
Where percent composition is used
Mass percent is the bridge between a formula and the lab bench. Analytical chemists compare a measured elemental analysis against the calculated composition to confirm a compound's identity or spot an impurity. It underpins empirical formula work, where experimental percentages are converted back into whole-number atom ratios. It also drives practical dosing, such as reading how much nitrogen a fertiliser labelled with an N-P-K rating actually delivers per kilogram.