JEE Main & Advanced Chemistry Equilibrium / साम्यावस्था Sample Paper Topic Test - Chemical Equilibrium

  • question_answer
    At temperature T, a compound \[A{{B}_{2(g)}}\] dissociates according to the reaction\[2A{{B}_{2(g)}}2A{{B}_{(g)}}+{{B}_{2(g)}}\] with a degree of dissociation x, which is small compared with unity. The expression for \[{{K}_{p}},\] in terms of x and the total pressure, P is

    A) \[\frac{P{{x}^{3}}}{2}\]         

    B) \[\frac{P{{x}^{2}}}{3}\]

    C) \[\frac{P{{x}^{3}}}{3}\]

    D) \[\frac{P{{x}^{2}}}{2}\]

    Correct Answer: A

    Solution :

    [a] \[\underset{1}{\mathop{2A{{B}_{2(g)}}}}\,\underset{0}{\mathop{2A{{B}_{(g)}}}}\,+\underset{0}{\mathop{{{B}_{2(g)}}}}\,\]
    Initially    
    At equilibrium \[\left( 1-x \right)~\]            x          x/2
    Total no. of moles \[=(1-x)+x+\frac{x}{2}=1+\frac{x}{2}=\frac{2+x}{2}\]
    Partial pressure = mole fraction x total pressure
    Applying \[{{K}_{p}}=\frac{p_{AB}^{2}\times {{p}_{{{B}_{2}}}}}{p_{A{{B}_{2}}}^{2}}\]
    \[=\frac{{{\left( \frac{x}{\frac{2+x}{2}}\times p \right)}^{2}}\times \left( \frac{\frac{x}{2}}{\frac{2+x}{2}}\times p \right)}{\left( \frac{1-x}{\frac{2+x}{2}}\times p \right)}=\frac{P{{x}^{3}}}{(2+x){{(1-x)}^{2}}}\]
    Since x << 1 so \[{{(1-x)}^{2}}\] can be neglected and (2 + x) can be taken as 2.
    \[\therefore \,{{K}_{P}}=\frac{P{{x}^{3}}}{2}\]


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