NEET AIPMT SOLVED PAPER SCREENING 2008

  • question_answer
    The dissociation equilibrium of a gas \[A{{B}_{2}}\] can be represented as \[2A{{B}_{2}}(g)2AB(g)+{{B}_{2}}(g)\] The degree of dissociation is 'x' and is small compared to 1. The expression relating the degree of dissociation (x) with equilibrium constant \[{{K}_{p}}\] and total pressure p is

    A) \[(2{{K}_{p}}/p)\]

    B)                        \[{{(2{{K}_{p}}/p)}^{1/3}}\]

    C) \[{{(2{{K}_{p}}/p)}^{1/2}}\]                        

    D) \[({{K}_{p}}/p)\]

    Correct Answer: B

    Solution :

    \[\begin{matrix}    {} & 2A{{B}_{2}}(g) & 2AB(g)+ & {{B}_{2}}(g)  \\    \text{Initial}\,\text{moles} & 1 & 0 & 0  \\    \text{At}\,\text{equ}. & 2(1-x) & 2x & x  \\ \end{matrix}\]where, x = degree of dissociation Total moles at equilibrium \[=2-2x+2x+x\]    \[=(2+x)\] So,          \[{{p}_{A{{B}_{2}}}}=\frac{2(1-x)p}{(2+x)}\]                 \[{{p}_{AB}}=\frac{2xp}{(2+x)}\]                 \[{{p}_{{{B}_{2}}}}=\frac{xp}{(2+x)}\]                 \[{{K}_{p}}=\frac{{{({{p}_{AB}})}^{2}}({{p}_{{{B}_{2}}}})}{{{({{p}_{A{{B}_{2}}}})}^{2}}}\] \[=\frac{{{\left( \frac{2xp}{2+x} \right)}^{2}}\left( \frac{x}{2+x}p \right)}{\left( \frac{2(1-x)}{(2+x)}p \right)}\] \[=\frac{{{x}^{3}}p}{(2+x){{(1-x)}^{2}}}\] \[[\because x<<<1and\,2,\,so\,(1-x)\approx 1,(2+x)\approx 2]\] \[=\frac{{{x}^{3}}p}{2}\] \[x={{\left( \frac{2{{K}_{p}}}{p} \right)}^{1/3}}\]


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