Which one of the three would have the lowest number of carbon atoms in its molecule ? Justify your answer

Question

The melting point of three members X, Y, and Z of a homologous series of hydrocarbons are -180°C, -140°C and -30°C respectively.

Which one of the three would have the lowest number of carbon atoms in its molecule ? Justify your answer

Answer 1

The homologue with lower number of C-atoms in its molecule has lower melting point. Therefore, compound X has the lowest number of carbon atoms in its molecule. It is clear from the given values of melting points in which -180°C is lowest.