UPSC Prelims 2001 Q118: Science & Technology – Chemistry
Which one of the following is the correct sequence in increasing order of molecular weights
of the hydrocarbons?
- (a) Methane, ethane, propane and butane
- (b) Propane, butane, ethane and methane
- (c) Butane, ethane, propane and methane
- (d) Butane, propane, ethane and methane
Answer: (a) Methane, ethane, propane and butane
Explanation
Hydrocarbons in increasing order of molecular weight
| Hydrocarbon | Formula and molecular weight |
|---|---|
| Methane | CH₄ — 16 |
| Ethane | C₂H₆ — 30 |
| Propane | C₃H₈ — 44 |
| Butane | C₄H₁₀ — 58 |
These are alkanes (CₙH₂ₙ₊₂). Each step adds a CH₂ group of mass 14, so the order of increasing molecular weight is methane, ethane, propane, butane.
Answer: (a) Methane, ethane, propane and butane.
Why each option is right or wrong
- (a) CORRECT. 16, 30, 44, 58 — increasing order.
- (b) Incorrect. This order is jumbled.
- (c) Incorrect. Butane is the heaviest, not the lightest.
- (d) Incorrect. This is decreasing order.
Key point
LPG is mainly propane and butane; CNG is mainly methane.
Remember
Methane 16 < ethane 30 < propane 44 < butane 58.
Common mistakes
- Choosing the decreasing order (d) by misreading.
- Not knowing the alkane formulas (CnH2n+2).
- Thinking the order of names is random.
Exam tip
Meth-, eth-, prop-, but- = 1, 2, 3, 4 carbons.
Sources
- NCERT Class X — Science, chapter 'Carbon and its Compounds'
- NCERT Class XI — Chemistry Part II, chapter 'Hydrocarbons'