Question

What is the difference in molar mass of Undecane and Dodecane?

• A. 10 $\text{g mol}^{-1}$
• B. 20 $\text{g mol}^{-1}$
• C. 140 $\text{g mol}^{-1}$
• D. 14 $\text{g mol}^{-1}$

Hint:

Members of ANY homologous series (alkanes, alcohols, amines, etc.) always differ by a $-\text{CH}_2-$ unit, meaning a constant mass difference of exactly 14 g/mol! No complex calculations are needed.

Answer 1

The Correct Option is D

Step 1: Understanding the Question:
The question asks for the numeric difference in the molar mass between two specific consecutive alkanes: undecane and dodecane.

Step 2: Detailed Explanation:
Undecane and dodecane are sequential members of the alkane homologous series.
- Undecane is an alkane with 11 carbon atoms. Its formula is $\text{C}_{11}\text{H}_{24}$.
- Dodecane is an alkane with 12 carbon atoms. Its formula is $\text{C}_{12}\text{H}_{26}$.
By the fundamental definition of any homologous series in organic chemistry, every successive member differs from the previous one by exactly one methylene group ($-\text{CH}_2-$).
Therefore, the difference in their molecular formulas is exactly one Carbon atom and two Hydrogen atoms ($1 \text{ C} + 2 \text{ H}$).
Let's calculate the molar mass of a $-\text{CH}_2-$ group:
Mass of Carbon (C) = $12 \text{ g/mol}$
Mass of Hydrogen (H) = $1 \text{ g/mol}$
Difference in mass = $(1 \times 12) + (2 \times 1) = 12 + 2 = 14 \text{ g/mol}$.
(Alternatively, manually calculating: Molar mass of undecane = $(11 \times 12) + 24 = 156 \text{ g/mol}$. Molar mass of dodecane = $(12 \times 12) + 26 = 170 \text{ g/mol}$. The difference is $170 - 156 = 14 \text{ g/mol}$).

Step 3: Final Answer:
The difference is 14 $\text{g mol}^{-1}$, matching option (d).