question_answer 109)

  The ionisation constant of an acid,\[{{K}_{a}}\], is the measure of strength of an acid. The \[{{K}_{a}}\] values of acetic acid, hypochlorous acid and formic acid are \[1.74\times {{10}^{-5}}\],\[3.0\times {{10}^{-8}}\]and \[1.8\times {{10}^{-4}}\]respectively. Which of the following orders of pH of 0.1 mol \[d{{m}^{-3}}\] solutions of these acids is correct? (a) Acetic acid > hypochlorous acid > formic acid                (b) Hypochlorous acid > acetic acid > formic acid (c) Formic acid > hypochlorous acid > acetic acid (d) Formic acid > acetic acid >hypochlorous acid

Answer:

  (d) \[[{{H}^{+}}]=\sqrt{C{{K}_{a}}}\] Thus, greater is the value of \[{{K}_{a}}\] more will be the acidic character. \[\therefore \]\[\frac{\begin{align}   & \underset{1.8\times {{10}^{-4}}}{\mathop{Formic\,acid}}\,>\underset{1.8\times {{10}^{-5}}}{\mathop{Acetic\,acid}}\,>\underset{3\times {{10}^{-8}}}{\mathop{Hypochlorous\,acid}}\, \\ \end{align}}{Decrea\sin g\,Acidic\,Character}\]