Question

What is the common formula for the alkanes ?

• A. $C_nH_{2n}$
• B. $C_nH_{2n+2}$
• C. $C_nH_{2n-2}$
• D. $C_nH_n$

Hint:

Remember the hydrocarbon formula rules:
- Alkanes (single bonds): $C_nH_{2n+2}$
- Alkenes (one double bond): $C_nH_{2n}$
- Alkynes (one triple bond): $C_nH_{2n-2}$

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Alkanes are saturated hydrocarbons consisting entirely of single covalent bonds between carbon and hydrogen atoms.
They form a homologous series where each member differs from the next by a $-\text{CH}_2-$ unit, sharing a common general molecular formula.

Step 3: Detailed Explanation:
Let us analyze the general molecular formulas for different hydrocarbon classes:
- Alkanes (Saturated Hydrocarbons): Because carbon has a valency of 4 and forms only single bonds in alkanes, each carbon atom ($n$) is bonded to two hydrogen atoms, with the two terminal carbons bonded to an additional hydrogen atom each.
This results in the general formula:
\[ C_nH_{2n+2} \]
For example: Methane ($CH_4$, $n=1$), Ethane ($C_2H_6$, $n=2$), and Propane ($C_3H_8$, $n=3$).
- Let us review the other options:
- Option (A) $C_nH_{2n$:} General formula for alkenes (unsaturated hydrocarbons containing one double bond) or cycloalkanes.
- Option (C) $C_nH_{2n-2$:} General formula for alkynes (unsaturated hydrocarbons containing one triple bond).
- Option (D) $C_nH_n$: Represents specific unsaturated cyclic compounds (such as Benzene, $C_6H_6$), but is not a general formula for an entire aliphatic class.
Therefore, $C_nH_{2n+2}$ is the correct formula for alkanes.

Step 4: Final Answer:
The common formula for the alkanes is $C_nH_{2n+2}$.