Organic Chemistry Question!

propane is quite unreactive compared to other organics.
however, propane is combustible.
when burnt in air, it gives carbon dioxide and water as only products of complete combustion:
C3H8 + 5O2 ------> 3CO2 + 4H2O

1.
burn propane in excess pure oxygen.
pass the product gas mixture into lime water. formation of milky suspension suggests the presence of carbon dioxide in gas.
as pure oxygen does not contain carbon (in form of carbon dioxide), carbon must come from propane.

2.
similarly, burn propane in excess pure DRY oxygen.
cool down the gas with condenser / cold finger etc., and dip a dry blue cobalt(II) chloride paper onto liquid condensed.
color changing from blue to pink suggests presence of water.
as dry oxygen contains no hydrogen (in form of water), hydrogen comes from (combustion of) propane.


of course there're a lot more other (advance) methods, like mass spectrometry, IR spectrometry and NMR spectrometry... but keep it simple.

note that these questions are related to some stoichiometric calculations, asking you to find C, H (and sometimes O) ratio in a compound.
water can be absorbed by conc. sulfuric acid, calcium chloride, calcium oxide or dry silica gel beads.