Answer 1

The general formula for the homologous series whose first member is ethene is C_nH_2n.

The two consecutive members (2nd and 3rd) of this series are: 

a. CH₃CH=CH₂ (Propene); where n is 3 

b. CH₃CH₂CH=CH₂(1-Butene) or CH₃CH=CHCH₃ (2-Butene); where n is 4 

The molecular formulae are C₃H₆ and C₄H₈ respectively.