Write the molecular formula of the 2nd and 3rd member of the homologous series whose first member is ethane.
Ethane (C2H6) is an alkane with 2 carbon atoms. To obtain homologues, add a –CH2 unit to the previous member of the series.
So, C2H6 + (–CH2) → C3H8 (second member of the homologous series)
C3H8 + (–CH2) → C4H10 (third member of the homologous series)
C3H8 is propane and C4H10 is butane.