Percent composition is just what it sounds like: it is finding the percentages of elements that make up a chemical compound. For this post, let’s use sulfuric acid, whose chemical formula is H2SO4. Now, it may look like the atoms are arranged in a ratio of 2 hydrogen to 1 sulfur to 4 oxygen, because that is correct. What isn’t correct, though, is the thought that the percent composition of hydrogen here is (2*100)/ 7= 28% of the entire formula. This is because hydrogen, sulfur, and oxygen all have very, very different masses, which must be taken into account, because the more mass of an element you have, the more atoms of it there are that can form sulfuric acid.

To solve this, we go by the total number of atoms there are in a particular mass of an element. This is called a mole, and is similar to a dozen; while a dozen counts 12 objects at a time, a mole counts 6.022*1023 atoms at a time. While that number, named after its inventor Avogadro, may seem like a big and formidable number, it in fact is, but atoms make up for it by being so tiny. We also use what are called molecular masses, which tell us the mass of one mole of an element or compound. The mole mass of an element is determined by its atomic mass; for example, an element with atomic mass of 5.004 means that one mole of element weights 5.004 grams. With compounds, you would use the sum of all the elements’ atomic masses, multiplied by whatever subscript they have. So finally, let’s do some math.

Hydrogen has atomic mass of 1.008, so that is 1.008 grams in one mole of H . For sulfur, you have 32.06 grams per mole. For oxygen, 16.00 g/mol. Multiply by subscripts to get:
H: 2(1.008g/mol) = 2.016g/mol.
S: 1(32.06g/mol) = 32.06g/mol.
O: 4(16.00g/mol) = 64.00g/mol.

These sum to 98.076 g/mol and round to 98.08g/mol. Since the mole counts the same number of atoms, there are 2 moles of H for every 1 mol of S and for every 4 mol of O. Now that their different masses are all taken into account, we can get rid of the /mol part, and we can divide the mass/amount of each element over the mass/amount of the whole compound, times 100, to get percent compositions:

H: (2.016g of H  / 98.076g of H2SO4)*100 = 2.056%, or 2.056g H per 100g H2SO4.
S: (32.06g S / 98.076g H2SO4)*100 = 32.69%, or 32.69g S per 100g H2SO4.
O: (64.00g O / 98.076g H2SO4)*100 = 65.26%, or 65.26g O per 100g H2SO4.

If you would like, there is this page with more examples on finding percent compositions. Thank you for reading.