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 Concept:
Alkanes form a homologous series where each successive member differs from the previous one by a $-CH_2-$ group.

Step 2: Formula Application:
Molar mass of $-CH_2-$ unit = $\text{Mass of } C + 2 \times (\text{Mass of } H)$.

Step 3: Explanation:
1. Undecane is $C_{11}H_{24}$. 2. Dodecane is $C_{12}H_{26}$. The difference is exactly one $CH_2$ unit. Mass = $12 + 2(1) = 14$ g mol$^{-1}$.

Step 4: Final Answer:
The difference is 14 g mol$^{-1}$.