MCQ
At temperature $T$ a compound $AB_2(g)$ dissociates according to the reaction with a degree of dissociation $'x'$ which is very small compared to unity. The value of $x$ is 

$2A{B_2}(g) \rightleftharpoons 2AB(g) + {B_2}(g)$

  • A
    $\sqrt {\frac{{2{K_P}}}{P}} $
  • $\sqrt[3]{{\frac{{2{K_P}}}{P}}}$
  • C
    $\sqrt[3]{{\frac{{{K_P}}}{P}}}$
  • D
    $\sqrt[3]{{{K_P}}}$

Answer

Correct option: B.
$\sqrt[3]{{\frac{{2{K_P}}}{P}}}$
b
$2A{B_2}(g) \leftrightarrow 2AB(g) + {B_2}(g)$

$1$                         $0$                $0$

Initially

$(1-x)$               $x$               $\frac {x}{2}$

At equilibrium

Total moles at equilibrium $=$ 

$1-x+x+\frac{1}{2}=1+\frac{x}{2}=1$

$[\because \,{\text{x}}$ is small incomparision to unity $]$

${P_{{\text{A}}{{\text{B}}_2}}} = (1 - {\text{x}}){\text{P}}\quad {P_{{\text{AB}}}} = {\text{xP}}\quad {{\text{p}}_{{{\text{B}}_2}}} = \frac{{{\text{xP}}}}{2}$

$\mathrm{K}_{\mathrm{p}}=\frac{\mathrm{x}^{3} \mathrm{P}^{3}}{2(1-\mathrm{x})^{2} \mathrm{P}^{2}}=\frac{\mathrm{x}^{3} \mathrm{P}}{2} \quad[\because(1-\mathrm{x}) \approx 1]$

$K_{p}=\frac{x^{3} P}{2}$

$x^{3}=\frac{2 K_{p}}{P}$

${x}=\sqrt{\frac{2 K_{p}}{P}}$

$(2)$ is correct answer.

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