A) \[{{10}^{-7}}\]
B) \[{{10}^{-5}}\]
C) \[{{10}^{-10}}\]
D) \[{{10}^{-8}}\]
Correct Answer: C
Solution :
Key Idea We know that \[[{{H}^{+}}]=\sqrt{{{K}_{a}}\,.\,C}\] So, \[pH=-\log \,\sqrt{{{K}_{a}}\,.\,C}\,(pH=-\log \,[{{H}^{+}}])\] Here, pH = 5, C = 1 M Therefore, \[pH=-\log \,\sqrt{{{K}_{a}}C}\] \[5=-\log \,\sqrt{{{K}_{a}}\times 1}\] \[=-\frac{1}{2}\log \,{{K}_{a}}\] \[\log \,{{K}_{a}}=-10\] \[{{K}_{a}}={{10}^{-10}}\] Hence, dissociation constant \[({{K}_{a}})={{10}^{-10}}\].
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