If we take acetone, H_3C-C(=O)CH_3, or butanone, C_4H_8O, we can see that these simple formula do conform to the general formula given above. The carbonyl group, C=O, represents a degree of unsaturation; that is 2 hydrogens less than that possessed by the comparable alkane. For propanone, with 3 carbon atoms, the comparative saturated species is propane, C_3H_8.
A degree of unsaturation represents a double bond (to C=C, or C=O, or C=N, OR a ring structure (i.e. the carbon termini bind to each other, cf. cyclopropane, C_3H_6, compared to propane, C_3H_8. If there is a nitrogen in the molecule (as often occurs in organic chemistry), we subtract NH from the formula before we calculate the degree of unsaturation.
