Write the molecular formula of the 2nd and 3rd member of the homologous series whose first member is methane.

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Solution

The organic compounds constituting only Carbon ( $C$ ) and Hydrogen ( $H$ ) atoms are called hydrocarbons.
The organic compounds containing carbon atoms have a single bond between two carbon atoms are called saturated hydrocarbons.
Example: Methane ( ${CH}_{4}$ )
A homologous series is a collection of molecules with the same chemical characteristics and similar functional groups that differ by $- {CH}_{2}$ group from one another
The difference between ${CH}_{4}$ and $C_{2} H_{6}$ is $- {CH}_{2}$ unit, whereas Ethane ( $C_{2} H_{6}$ ) and propane ( $C_{3} H_{8}$ ) is also differed by $- {CH}_{2}$ unit. As a result, both are homologous.
Therefore, the molecular formula of the 2nd and 3rd members of the homologous series is Ethane $C_{2} H_{6}$ and Propane $C_{3} H_{8}$ .

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