For % Composition, when to use mass method and when to use formula method Any hint or trick?
Hard to know what you're asking here, but a tip is that the balance is always oxygen. What do I mean?
Suppose you had an unknown sample composed of C, H, halogen, and oxygen. To get the elemental composition, you could burn it in an electric furnace, and measure the carbon dioxide given off, and also the water given off very precisely. This is what is done routinely. By other means you could determine the halogen content.
But how could you determine the oxygen content? We are surrounded by oxygen gas that could potentially contaminate your results. Now there are some means of measuring oxygen, but they are rare. So here's the tip. If you're given a percentage composition that does not add up to 100% (and why should it add to 100%?), the balance, the missing percentage, is for oxygen content.