JEE Main & Advanced Sample Paper JEE Main - Mock Test - 44

  • question_answer
    \[15\text{ }mL\]of gaseous hydrocarbon was required for complete combination. \[357\text{ }mL\]of air (\[21%\]oxygen by volume) and gaseous products occupied \[327\text{ }mL\](all volumes being measured at STP). The molecular formula of the hydrocarbon is

    A) \[{{C}_{2}}{{H}_{6}}\]   

    B)                    \[{{C}_{2}}{{H}_{4}}\]                 

    C) \[{{C}_{3}}{{H}_{6}}\]                

    D)        \[{{C}_{3}}{{H}_{8}}\]

    Correct Answer: D

    Solution :

    [d] Let the formula of hydrocarbon A is CxHy. \[{{C}_{x}}{{H}_{y}}(g)+\left( x+\frac{y}{4} \right){{O}_{2}}(g)\xrightarrow{{}}xC{{O}_{2}}(g)+\frac{y}{2}{{H}_{2}}O(l)\]\[1\,mL\,\,\,\,\,\,\,\,\,\,\,\left( x+\frac{y}{4} \right)mL\]                       \[xmL\,\,\,-\] \[15\,mL\,\,\,\,\,\,\,\,\,\,\,15\left( x+\frac{y}{4} \right)mL\]  \[15x\,mL-\] Volume of \[{{O}_{2}}\frac{357\times 21}{100}=75\,mL;\] volume of \[{{N}_{2}}\]                                     \[=357-75=282\text{ }mL\] Volume of \[C{{O}_{2}}=327-282=45mL\] \[\therefore \,\,\,\,\,\,\,\,15x=45,\,\,x=3\] \[\therefore \,\,\,\,\,\,\,\,\left( \begin{align}   & 15\left( x+\frac{y}{4} \right)=75 \\  & x+\frac{y}{4}=5 \\ \end{align} \right)\] \[3+\frac{y}{4}=5,\]On solving we get\[y=8\]. Formula of \[(A)={{C}_{3}}{{H}_{8}}\]


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