Expert Reply
Sat December 29, 2012
Petrol has no definite formula because it is a mixture of several compounds. Petrol is a mixture of benzene, several hydrocarbons and iso-octanes. Most of the hydrocarbons in petrol are considered to be harmful substances and must be distilled for effectiveness. However, the largest component of almost all commercial petrols is isooctane, which has the formula C8H18, and its isomers, which have the same formula of disel.
 
Coal is not actually a compound, but rather a mixture of compounds and so it does not have a chemical formula.

However depending on the type of coal, it is composed of between 50-100% carbon, by mass, with the rest being Hydrogen, Nitrogen, Oxygen, and trace amounts of Sulfur. 
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