What is the general formula for a saturated hydrocarbon?

• A. CnH2n
• B. CnH2n+2
• C. CnH2n+1
• D. CnH2n-2

Answer: (B)

In a saturated hydrocarbon, every carbon forms only single bonds and each carbon has four bonds total. For a molecule with n carbon atoms, there are n−1 carbon–carbon bonds in a straight chain. Each carbon–carbon bond uses two valence slots, so the hydrogens must supply the remaining valences: total valence needed is 4n, subtract 2(n−1) for the C–C bonds, giving 4n − 2(n−1) = 2n + 2 hydrogens. Therefore the general formula is CnH2n+2. This matches simple examples: methane has 1 carbon and 4 hydrogens, ethane has 2 carbons and 6 hydrogens, and so on. If you introduce rings or multiple bonds, hydrogens decrease by 2 for each unit of unsaturation, so those would not be saturated hydrocarbons.