Answer 1

The general formula for the homologous series whose first member is ethyne is C_nH_{2n - 2.}

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

a. CH₃C≡CH (Propyne); where n is 3 

b. CH₃CH₂C≡CH(1-Butyne) or CH₃C≡CCH₃ (2-Butyne); where n is 4 

The molecular formulae are C₃H₄ and C₄H₆, respectively.