Question

The first three members of a homologous series are $C{H_4}$, ${C_2}{H_6}$, ${C_3}{H_8}$ the fifth member of his series will be:
A) ${C_5}{H_{10}}$
B) ${C_5}{H_{14}}$
C) ${C_5}{H_{12}}$
D) ${C_5}{H_8}$

Answer 1

As we know that in organic chemistry, hydrocarbons are an important topic. The hydrocarbons are majorly classified as three groups. There are alkane, alkene and alkyne. The alkane means carbon-carbon single bond. The alkene has a carbon-carbon double bond. The alkyne means carbon-carbon having triple bond in the molecule. The full form of IUPAC is the International Union of Pure and Applied Chemistry. It is a World Wide organisation for the nomenclature of the new elements in the periodic table and chemical molecules. The IUPAC has certain rules and regulations for the naming of the organic compound. In that method only we named the organic molecules in World level. The naming of any organic molecule depends on the parent saturated hydrocarbon.

Complete answer:
The general homologous formula for the alkane is ${{\text{C}}_{\text{n}}}{{\text{H}}_{{\text{2n + 2}}}}$.
Methane is the first member of the alkane homologous series. The molecular formula of methane is $C{H_4}$.
Ethane is the second member of the alkane homologous series. The molecular formula of ethane is ${C_2}{H_6}$.
Propane is the third member of the alkane homologous series. The molecular formula of propane is ${C_3}{H_8}$.
Butane is the fourth member of the alkane homologous series. The molecular formula of butane is ${C_4}{H_{10}}$.
Pentane is the fifth member of the alkane homologous series. The molecular formula of pentane is ${C_5}{H_{12}}$.
According to the above discussion, we conclude pentane is the fifth member of the alkane homologous series. The molecular formula of pentane is ${C_5}{H_{12}}$.

Hence, the option C is the correct answer.

Note:
We need to know the conversion of one type of hydrocarbon to another hydrocarbon by oxidation and reduction. The oxidation of alkane to give alkene. The oxidation of alkene to give alkyne. The reduction of alkyne to give alkene. The reduction of alkene to give alkane. It has some general formulas. The general formula of alkane is ${{\text{C}}_{\text{n}}}{{\text{H}}_{{\text{2n + 2}}}}$. The general formula of alkene is ${{\text{C}}_{\text{n}}}{{\text{H}}_{{\text{2n}}}}$. The general formula of alkyne is ${{\text{C}}_{\text{n}}}{{\text{H}}_{{\text{2n - 2}}}}$. The oxidation means the addition of oxygen or removal of hydrogen or the loss of electrons in the reactant in chemical reaction. The reduction means the removal of hydrogen or the addition of oxygen or the gain of electrons in the chemical reaction.