Question

What is the difference in molar mass of a member of a homologous series from its neighboring members in grams per mole?

• A. 14
• B. 18
• C. 30
• D. 25

Hint:

No matter how complex the functional group is—whether it's an alcohol, carboxylic acid, aldehyde, or alkane—the structural bridge from one neighbor to the next is always just a single $-\mathrm{CH_2}-$ group. Its molecular mass is always $12 + 2 = 14$.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
Find the molar mass difference between consecutive members of a homologous series.

Step 2: Key Formula or Approach:
Adjacent homologs differ by one -CH₂- (methylene) repeating unit.

Step 3: Detailed Explanation:
Mass of -CH₂- = 12 (C) + 2×1 (H) = 14 g mol⁻¹. For example, methane (16) and ethane (30) differ by 14. This constant applies across all homologous series.

Step 4: Final Answer:
The difference is 14, option (A).