Official Answer Key & Detailed Solution
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Official UPSC Answer KeyUPSC Prelims 2001 • Science and Technology
Correct Answer: Option (A) — Methane, ethane, propane and butane
Concept & Option Breakdown
Standard Reference VerifiedIn the alkane homologous series ($C_nH_{2n+2}$):
- Methane ($CH_4$) = $16\text{ g/mol}$
- Ethane ($C_2H_6$) = $30\text{ g/mol}$
- Propane ($C_3H_8$) = $44\text{ g/mol}$
- Butane ($C_4H_{10}$) = $58\text{ g/mol}$ Thus, the increasing order is Methane < Ethane < Propane < Butane.
Elimination Heuristic & Exam Approach
Number of carbons increases systematically: $C_1$ (Methane) < $C_2$ (Ethane) < $C_3$ (Propane) < $C_4$ (Butane).
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