Themes > Science > Chemistry > Miscellenous > Help file Index > Stoichiometry > Percent composition The percentage composition of a molecule is just the mass percentage of each element in the molecule. To get the mass percentage, simply divide the total mass of each element in the molecule by the molecular weight of the molecule and multiply by 100% % element = (mass element/molecular weight) *100% For example, we can work out the mass percentage of each atom in the molecule CaCO3, calcium carbonate. This molecule has 1 calcium atom, 1 carbon and 3 oxygen. The molecular weight is thus Mole weight CaCO3 = mole weight Ca + mole weight C + 3*(mole weight O) Mole weight CaCO3 = 40.08 g/mole + 12.01 g/mole + 3*(16.00 g/mole) Mole weight CaCO3 = 100.09 g/mole The mass percentage of each is now easy to figure out: % Ca = Mass Ca/100.09 g/mole *100% = 40.08/100.09 *100% = 40.04% % C = Mass C/100.09 g/mole *100% = 12.01/100.09 *100% = 12.00% % O = 3*Mass O/100.09 g/mole *100% = 3*16.00/100.09 *100% = 47.96% Example: What is the mass percentage of potassium in potassium dichromate, K2Cr2O7 Solution First, work out the molecular weight of potassium dichromate mole weight = 2*MWK + 2*MWCr = 7*MWO mole weight = 2*39.10 g/mole + 2*52.00 g/mole + 7*16.00 g/mole mole weight = 294.2 g/moleNext, just divide the mass of the potassium in the molecule by the molecular weight: %K = 2*(mass K)/294.2 g/mole * 100% %K = 2*(39.10 g/mole)/294.2 g/mole * 100% %K = 26.58% Information provided by: http://learn.chem.vt.edu