Question

The first three members of a homologous series are ${\mathrm{CH}}_4,{\mathrm{C}}_2{\mathrm{H}}_6,{\mathrm{C}}_3{\mathrm{H}}_8$ the fifth member

• A. ${\mathrm{C}}_5{\mathrm{H}}_{12}$
• B. ${\mathrm{C}}_5{\mathrm{H}}_8$
• C. ${\mathrm{C}}_5{\mathrm{H}}_{10}$
• D. ${\mathrm{C}}_5{\mathrm{H}}_{14}$

Answer 1

The correct option is A

${\mathrm{C}}_5{\mathrm{H}}_{12}$

Explanation for correct option:

A. ${\mathrm{C}}_5{\mathrm{H}}_{12}$

Homologous series:

• “The ability of carbon to form chains gives rise to a homologous series of compounds in which the same functional group is attached to carbon chains of different lengths.”
• It is given that the first three members of a homologous series are ${\mathrm{CH}}_4,{\mathrm{C}}_2{\mathrm{H}}_6,{\mathrm{C}}_3{\mathrm{H}}_8$. It is a homologous series of alkanes.
• Alkanes have a general molecular formula which is ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}+2}$.
• Thus, the fifth member of this series will be, $\mathrm{n}=5,{\mathrm{C}}_5{\mathrm{H}}_{(2\times 5+2)}={\mathrm{C}}_5{\mathrm{H}}_{12}$

Explanation for incorrect options:

As the fifth member of the homologous series is ${\mathrm{C}}_5{\mathrm{H}}_{12}$. Thus, options B, C, and D are incorrect.

Hence, the correct option is A i.e. ${\mathrm{C}}_5{\mathrm{H}}_{12}$.