There are two different ways to describe the composition of a compound: in terms of the number of its constituent atoms (like C2H6)and in terms of the percentages (by mass) of its elements. When showingthe constituent atoms of a molecule, you can either show the chemicalformula, which shows the real number of atoms in the molecule, like C2H6, or show the
empirical formula, which merely shows their relative amounts in a substance, so the above molecular formula would be expressed as CH3.
You can describe the composition of acompound in terms of the weights of its constituent elements bydetermining the percent composition of particular elements in themolecule. To calculate percent compositions, you would find the weightof each constituent atom, then figure out what percent of the totalmolecular weight it makes up. Consider ethanol, C2H5OH.Taking subscripts into consideration, you have 2 mols of carbon, 6 molsof hydrogen (5 + 1), and 1 mol of O. Now convert moles into grams foreach constituent element as well as for the entire molecule:
Mass of C = 2 
12.01
= 24.02 g
Mass of H = 6 
1.01
= 6.06 g
Mass of O = 1 
16.00
= 16.00 g
Mass of 1 mol of C2H5OH = 46.08 g
Now use the formula you learned above to find the percent compositions of the constituent elements:
Mass percent of C: 
100% = 52.14%
Mass percent of H: 
100% = 13.15%
Mass percent of O: 
100% = 34.77%
Not so bad, right?
