MCQ
One mole of a compound $AB$ reacts with one mole of a compound $CD$ according to the equation $AB + CD$ $\rightleftharpoons$ $AD + CB$. When equilibrium had been established it was found that $\frac{3}{4}$mole each of reactant $AB$ and $CD$ had been converted to $AD$ and $CB$. There is no change in volume. The equilibrium constant for the reaction is
  • A
    $\frac{9}{{16}}$
  • B
    $\frac{1}{9}$
  • C
    $\frac{{16}}{9}$
  • $9$

Answer

Correct option: D.
$9$
(d)                                   $AB$ + $CD$ $ \rightleftharpoons $ $AD$ + $CD$

mole at $t = 0$                     $1$          $1$         $0$         $0$

Mole at equilibrium    $\left( {1 - \frac{3}{4}} \right)$     $\left( {1 - \frac{3}{4}} \right)$   $ \rightleftharpoons $    $\left( {\frac{3}{4}} \right)$      $\left( {\frac{3}{4}} \right)$

                                           $0.25$        $0.25$      $ \rightleftharpoons $     $0.75$     $0.75$

${K_c} = \frac{{0.75 \times 0.75}}{{0.25 \times 0.25}} = \frac{{0.5625}}{{0.0625}} = 9$

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