There is but your mathematical skills need to be good in determining the number of permutations possible: Eg simple hydrocarbons such as methane, ethane and propane obviously dont have isomers, but butane has 2, pentane 3, hexane 6...and C20H42 would have 366 319 isomers! If other atoms are present, then the permutations increase further. Think of what a typical HSC question may ask, the exam committee would probably restrict it to a maximum of 5 carbons in the chain or associate one isomeral structural formula to another. Your question of isomers of dichloro propane would have a maximum of 3: 1,1-dichloro propane, 1,2-dichloro propane and 1,3-dichloro propane. Sorry "chousta" but you are incorrect as 2,3-dichloropropane is a redundant term for 1,2-dichloro propane. So there are only 3 for this example. It is best you quickly draw them out as you have about 1.8 minutes per mark to determine the answer, but for longer chains note any cyclic or double bond possibilties that may exist. Mr Z