question_answer 1)

  On the basis of the equation \[pH=-\log [{{H}^{+}}]\], the pH of \[{{10}^{-8}}mol\,d{{m}^{-3}}\] solution of \[HCl\] should be 8. However, it is observed to be less than 7.0. Explain the reason.

Answer:

  When HCl is dissolved in water, \[{{H}^{+}}\] ion contribution of water will also be taken since it is very dilute solution. \[{{[{{H}^{+}}]}_{Total}}={{[{{H}^{+}}]}_{HCl}}+{{[{{H}^{+}}]}_{Water}}\] \[={{10}^{-8}}+{{10}^{-7}}\] \[=11\times {{10}^{-8}}M\] \[pH=-\log [{{H}^{+}}]\] \[=-\log [11\times {{10}^{-8}}]=6.9586\]