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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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