What is the general formula for alkanes?

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

Answer: (A)

The main idea is that alkanes are saturated hydrocarbons with only single bonds, so each carbon forms four bonds in total. For an acyclic alkane with n carbon atoms, the carbon skeleton has n−1 carbon–carbon single bonds, using 2(n−1) of the valence bonds between carbons. The total valence from all carbons is 4n, so the remaining bonds must be to hydrogen: 4n − 2(n−1) = 2n + 2 hydrogens. This gives the general formula CnH2n+2, which matches known examples like methane (CH4), ethane (C2H6), and propane (C3H8). The other formulas don’t fit alkanes: CnH2n would describe unsaturated hydrocarbons or cycloalkanes with rings, and CnH2n−2 applies to alkynes (and some cycloalkynes), not to alkanes.