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A 20 mL sample of a gaseous hydrocarbon is ignited with 120 mL of oxygen. After the explosion a volume contraction of 60 mL is observed, and a further contraction of 60 mL occurs when the residual gas is treated with KOH solution. Identify the hydrocarbon.

1. C3H6
2. C3H8
3. C2H6
4. C4H10

Correct answer: C3H8

Solution

Initial volume = 20 + 120 = 140 mL. After explosion, volume = 140 - 60 = 80 mL (CO2 + excess O2, water condensed). KOH absorbs CO2 = 60 mL; remaining = 20 mL (excess O2). O2 consumed = 120 - 20 = 100 mL. For CxHy + (x+y/4)O2 -> x CO2 + y/2 H2O: 20(x+y/4) = 100 -> x+y/4=5; 20x = 60 -> x=3; y=8. So C3H8 (propane).

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