What is a homologous series ? Write the molecular formulae of the first two members of the alkane series.

Answer: A series of compounds having the same functional group and similar chemical properties, in which successive members differ by a –CH₂– unit. First two alkanes: CH₄ (methane) and C₂H₆ (ethane).

• Members of a homologous series can be written by one general formula; for alkanes it is \(\mathrm{C_nH_{2n+2}}\).
• Successive members differ by one –CH₂– group, i.e. by 14 u in molecular mass, and their physical properties change gradually.
• n = 1: CH₄ (methane); n = 2: C₂H₆ (ethane).