JEE Main & Advanced JEE Main Solved Paper-2016

  • question_answer
    At 300 K and 1 atm, 15 mL of a gaseous hydrocarbon requires 375 mL air containing 20% \[{{O}_{2}}\] by volume for complete combustion. After combustion the gases occupy 330 mL. Assuming that the water formed is in liquid form and the volumes were measured at the same temperature and pressure, the formula of the hydrocarbon is :- [JEE Main Solved Paper-2016 ]

    A) \[{{C}_{4}}{{H}_{10}}\]                  

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

    C) \[{{C}_{3}}{{H}_{8}}\]                    

    D) \[{{C}_{4}}{{H}_{8}}\]

    Correct Answer: C

    Solution :

                    Volume of \[{{N}_{2}}\]in air = 375 × 0.8 = 300 ml volume of \[{{O}_{2}}\]in air = 375 × 0.2 = 75 ml \[{{C}_{x}}{{H}_{y}}+\left( x+\frac{y}{4} \right){{O}_{2}}\xrightarrow[{}]{{}}xC{{O}_{2}}(g)+\frac{y}{2}{{H}_{2}}O(\ell )\] \[15\,ml\,15\left( x+\frac{y}{4} \right)\]                0                              0              15           - After combustion total volume \[330={{V}_{{{N}_{2}}}}+{{V}_{C{{O}_{2}}}}\] \[330=300+15x\] \[x=2\] Volume of \[{{O}_{2}}\]used \[15\left( x+\frac{y}{4} \right)=75\] \[x+\frac{y}{4}=5\]                          \[y=12\]  So hydrocarbon is\[={{C}_{2}}{{H}_{12}}\] none of the option matches it therefore it is a BONUS. Alternatively \[{{C}_{x}}{{H}_{y}}+\left( x+\frac{y}{4} \right){{O}_{2}}\xrightarrow[{}]{{}}xC{{O}_{2}}+\frac{y}{2}{{H}_{2}}O(\ell )\] 15                           \[15\left( x+\frac{y}{4} \right)\] 0                              0              15x         - Volume of \[{{O}_{2}}\]used \[15\left( x+\frac{y}{4} \right)=75\] \[x+\frac{y}{4}=5\] If further information (i.e., 330 ml) is neglected, option (3) only satisfy the above equation.


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