"Because of its double bond, an alkene has fewer hydrogens than an alkane with the same number of carbons … and is therefore referred to as unsaturated. Knowing this relationship, it’s possible to work backward from a molecular formula to calculate a molecule’s degree of unsaturation — the number of rings and/or multiple bonds present in the molecule. … The unknown therefore contains two double bonds, one ring and one double bond, two rings, or one triple bond. There’s still a long way to go to establish structure, but the simple calculation has told us a lot about the molecule. … Add the number of halogens to the number of hydrogens. … Ignore the number of oxygens. … Subtract the number of nitrogens from the number of hydrogens."