Dear Student,
The Answer to your queries are as follows-
Basically, LPG stands for Liquefied Petroleum Gas and this is not a chemical substance as it a chemical product. It is generally a mixture of mostly (C3H8) Propane and (C4H10) Butane, with minor amounts of other hydrocarbons from C2 to C5. Whereas the chemical formula of CNG is (CH4) – which means that it only contains one carbon atom along with four hydrogen atoms.
Regards