In order to calculate the percentage composition of a certain element, certain steps are followed:
1- You get the the molar mass of the component you wish to find its percentage in units of gm/mole.
2- You then get the molar of the whole given compound
3- You the divide the molar mass of the desired component by the total molar mass of the compound and then multiply the output by 100 to get the percentage.
As you can see, calculating the percentage is dependent on the mass and not the volume
