JEE Main & Advanced Sample Paper JEE Main - Mock Test - 12

  • question_answer
    Hydrolysis of an alkyl halide (RX) by dilute alkali \[[\overset{\bigcirc -}{\mathop{O}}\,H]\] takes place simultaneously by \[{{S}_{N}}2\] and \[{{S}_{N}}1\] pathways. A plot of \[-\frac{1}{[RX]}\frac{d[R-X]}{dt}\]vs \[[\overset{\bigcirc -}{\mathop{O}}\,H]\] is a straight line of the slope equal to \[2\times {{10}^{3}}\,\,mo{{l}^{1}}\,\,L{{h}^{-1}}\] and intercept equal to\[1\times {{10}^{2}}{{h}^{-1}}.\]. Calculate the initial rate \[(mole\,\,{{L}^{-1}}{{\min }^{-1}})\] of consumption of RX when the reaction is carried out taking \[1\,\,mol\,\,{{L}^{-1}}\]of RX and \[0.1\text{ }mol\text{ }{{L}^{-1}}\] of \[[\overset{\bigcirc -}{\mathop{O}}\,H]\] ions.

    A)  4          

    B)                5   

    C) 3

    D)   4

    Correct Answer: B

    Solution :

    [b] \[\frac{-d[RX]}{dt}={{k}_{2}}[RX]\,[\overset{\bigcirc -}{\mathop{O}}\,H]\]   (by \[{{S}_{N}}2\]path way) \[{{k}_{2}}\]= rate constant of \[{{S}_{N}}2\] reaction \[\frac{-d[RX]}{dt}={{k}_{1}}[RX]\]     (by \[{{S}_{N}}1\] path way) \[{{k}_{1}}=\] rate constant of \[{{S}_{N}}1\] reaction \[\frac{-d[RX]}{dt}={{k}_{2}}[RX]\,\,[\overset{\bigcirc -}{\mathop{O}}\,H]+{{k}_{1}}[RX]\] \[-\frac{1}{[RX]}\frac{d[RX]}{dt}={{k}_{2}}[\overset{\bigcirc -}{\mathop{O}}\,H]+{{k}_{1}}\] This is the equation of a straight line for \[-\frac{1}{[RX]}=\frac{d[RX]}{dt}\] vs \[[\overset{\bigcirc -}{\mathop{O}}\,H]\] plot with slope equal to \[{{k}_{2}}\] and intercept equal to \[{{k}_{1}},\] From question: \[{{k}_{2}}=2\times {{10}^{3}}mo{{l}^{-1}}L\,h{{r}^{-1}},\,{{k}_{1}}=1\times {{10}^{2}}h{{r}^{-1}}\] \[[RX]=1.0M\] and \[[\overset{\bigcirc -}{\mathop{O}}\,H]=0.1M\] Hence, \[\frac{-d[RX]}{dt}=2\times {{10}^{3}}\times 1\times 0.1+1\times {{10}^{2}}\times 1\] \[=300\,mol\,{{L}^{-1}}h{{r}^{-1}}\] \[=5\,mol\,{{L}^{-1}}\,{{\min }^{-1}}\]                      


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