UPSC Prelims 2001 · General Studies

Question 118: Science and Technology

Which one of the following is the correct sequence in increasing order of molecular weights of the hydrocarbons?

aMethane, ethane, propane and butane

bPropane, butane, ethane and methane

cButane, ethane, propane and methane

dButane, propane, ethane and methane

AnswerThe answer is (a).

The molecular weight of a hydrocarbon rises with the number of carbon atoms: methane CH4 is 16, ethane C2H6 is 30, propane C3H8 is 44 and butane C4H10 is 58. So the increasing order is methane, ethane, propane, butane. Remember that these are the first four alkanes, and each step adds a CH2 group of 14 units.

Why each option is right or wrong

(a) Right: 16, 30, 44, 58.
(b) Puts propane, butane before ethane and methane, which is out of order.
(c) Starts with butane, the heaviest.
(d) This is the exact reverse of the correct order.

Explanation: written by UPSC Grade, not by UPSC.

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Source: UPSC, Civil Services (Preliminary) Examination 2001, General Studies. UPSC's own paper isn't online; the words are from a published compilation. The paper on upsc.gov.in.