Which class follows the general formula CnH2n?

• A. Alkenes
• B. Alkanes
• C. Alkynes
• D. Arenes

Answer: (A)

The number of hydrogens relative to the number of carbons reveals the type of carbon–carbon bonding in the molecule. When a hydrocarbon has one double bond, the hydrogen count is reduced by two compared with the saturated alkane, giving the general formula CnH2n for alkenes. This pattern is seen in simple examples like ethene (C2H4) and propene (C3H6), where doubling the carbon count roughly doubles the hydrogen count.

Alkanes, with only single bonds, are saturated and follow CnH2n+2 (ethane C2H6, propane C3H8). Alkynes, with a triple bond, have fewer hydrogens, following CnH2n-2 (ethyne C2H2). Arenes, which are aromatic rings, obey a different trend due to ring and delocalized bonding; the typical monocyclic arenes fit formulas like CnH2n-6 (benzene is C6H6), not CnH2n.

So the class that matches the general formula CnH2n is alkenes.