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Question

# 10mL of gaseous hydrocarbon was mixed with 100mL of oxygen and exploded in a eudiometer tube. The volume of the residual gases was 95mL out of which 20mL was absorbed by caustic soda the remaining by pyrogallol. The molecular formula of the gas is

A

C2H2

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B

C2H6

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C

C3H6

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D

C4H8

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Solution

## The correct option is A C2H2 volume left after explosion and cooling i.e volume of CO2 formed + unused O2 = 95mL volume after introducing KOH, i.e volume of CO2 formed = 20mL volume absorbed by alkaline pyrogallol i.e volume of unused O2 = 95-20 = 75mL ∴ volume of O2 used = 100-75 = 25mL Applying the combustion equation CxHy + (x+y4)O2 → xCO2 + y2 H2O 10mL 10(x+y4)mL 10xmL Negligible volume Equating theoretical and experimented values 10x=20 and 10 + x+y4=25 x=2 10 + 2+y4=25 y=2. Thus the hydrocarbon is C2H2

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