CET Karnataka Engineering CET - Karnataka Engineering Solved Paper-2011

  • question_answer
    The following data were obtained during the first order decomposition of \[2A\,(g)\to B\,(g)+C\,(s)\] at a constant volume and at a particular temperature
    S.no. Time Total pressure in Pascal
    1. At the end of 10 min 300
    2. After completion 200
    The rate constant in \[mi{{n}^{-1}}\] is

    A)  0.0693                                 

    B)  69.3

    C)  6.93                                      

    D)  \[6.93\times {{10}^{-4}}\]

    Correct Answer: A

    Solution :

    \[\underset{2-2x}{\mathop{2A\,(g)}}\,\xrightarrow{{}}\,\underset{x}{\mathop{B\,(g)}}\,+C\,(s)\] At the end of reaction, only 1 mole of gas is present whose pressure is 200 Pascal. \[\therefore \] At the beginning of the reaction 2 moles of gas should have a pressure of 400 Pascal. After time 10 min No. of moles present,                 \[2-2x+\text{ }x=2-x\] The pressure of 2 moles = 400 \[\therefore \]  \[400-x=300\] \[\therefore \]                  \[x=100\] \[\therefore \] Pressure due to \[2-2x\] moles of A                 \[=400-200=200\] \[\therefore \]       \[k=\frac{2.303}{t}\log \left( \frac{a}{a-x} \right)=\frac{2.303}{10}\log \left( \frac{400}{200} \right)\]                 \[=\frac{2.303}{10}\log \,\,2=\frac{0.693}{10}\]                 \[=0.0693\,{{\min }^{-1}}\]


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