Question

$0.20g$ of a hydrocarbon on combustion gave $0.66g$ of $C{O_2}$. The percentage of hydrogen in the hydrocarbon is about
(A) $35\%$
(B) $47\%$
(C) $10\%$
(D) $25\%$

Answer 1

At first we should calculate the mass of carbon dioxide produced by the combustion. We have been given a mass of hydrogen for the combustion process. Then the mass of hydrogen in the given hydrocarbon will be the difference in the mass of hydrocarbon and the mass of carbon corresponding to the combustion process.

Complete step by step solution:
We have been given mass of hydrocarbon to be $0.20\;g$ of a hydrocarbon
Mass of carbon dioxide $= 0.66\;g$
If the molecular weight of carbon dioxide is $44\;g$ and the mass of carbon is $12$
Than for $0.66\;g$ of carbon dioxide have x gram of carbon
Then the mass of carbon $= \dfrac{{12}}{{44}} \times 0.66$
mass of carbon $= 0.18\;g$
We have been given the total mass of hydrocarbon as $0.20\;g$
Mass of hydrogen $= 0.2 - 0.18$
Mass of hydrogen $= 0.02\;g$
This accounts for $10\%$ hydrogen in the hydrocarbon
Therefore the percentage of hydrogen in a hydrocarbon is $10\%$
Hence the correct answer is option C.

Note:
The hydrocarbon here contained the rest amount of carbon in the hydrocarbon. The direct percentage can be found out by multiplying by a hundred. The percentage of the elements are taken out to form the formula of the compound formed.