we all know that water is made of oxygen and hydrogen specifically one oxygen atom and two hydrogen atoms hence the formula H2O but how does water break down into its elements by mass we know that we could separate two water molecules into one oxygen molecule and two hydrogen molecules by electrolysis and this is the balanced chemical equation for that process so if we cause 100 G of water to undergo electrolysis what masses of oxygen gas and hydrogen gas should we expect to get this is easy to figure out if we find the percent composition of water which tells us the percentage by mass of each element in the molecule in order to do this we need to know the formula mass of a water molecule well each each hydrogen atom weighs one atomic mass unit and the oxygen atom weighs 16 atomic mass units so one water molecule weighs 18 atomic mass units we can break this down into percentages for each element the two hydrogen atoms weigh a total of two atomic mass units out of the 18 for the whole molecule so that makes water a little over 11% hydrogen by mass the oxygen atom weighs 16 atomic mass units out of the 18 for the whole molecule so water is just under 89% Oxygen by mass that means for 100 gram of water that could undergo electrolysis we would expect to get 11.1 gram of hydrogen gas and 88.9 gam of oxygen gas let's try something a little trickier vitamin C has the formula C6 h806 what is the percent composition of this compound well the whole thing adds up to 176 atomic mass units six carbon atoms give a mass of 72 so vitamin C is 40.9% carbon by mass the eight hydrogen atoms add up to eight so vitamin C is 4.5% hydrogen by mass and six oxygen atoms give a mass of 96 so vitamin C is 54.6% Oxygen by mass as we would expect these percentages add up to 100 to show that all the mass of the molecule is accounted for by these three elements