MCQ
The hydrogen ion concentration of a $0.006\,M$ benzoic acid solution is $({K_a} = 6 \times {10^{ - 5}})$
  • A
    $0.6 \times {10^{ - 4}}$
  • $6 \times {10^{ - 4}}$
  • C
    $6 \times {10^{ - 5}}$
  • D
    $3.6 \times {10^{ - 4}}$

Answer

Correct option: B.
$6 \times {10^{ - 4}}$
(b) Initial concentration $= 0.006\, M$ $==>$ ${K_a} = 6 \times {10^{ - 5}}$
equilibrium reaction.

$\mathop {\mathop {{C_6}{H_5}COOH}\limits_C }\limits_{C(1 - a)\,\,\,} $$ \rightleftharpoons $$\mathop {\mathop {{C_6}{H_5}CO{O^ - }}\limits_O }\limits_{Ca} + \mathop {\mathop {{H^ + }\,\,\,\,\,\,\,}\limits_{O\,\,{\rm{initial}}} }\limits_{Ca\,\,\,\,\,\,\,\,\,\,\,\,\,\,} $

${K_a} = C{\alpha ^2}$ for weak electrolyte

$\alpha = \sqrt {\frac{{{K_a}}}{C}} $; $\alpha = \sqrt {\frac{{6 \times {{10}^{ - 5}}}}{{0.006}}} $; $\alpha = {10^{ - 1}}$

$ = 0.006 \times {10^{ - 1}}$$ = 6 \times {10^{ - 4}}\,M$.

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