1. Tardigrade
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  3. Chemistry
  4. The dissociation constant of HCN is 5× 10-8 . The pH of the solution prepared by mixing 1.5 moles of HCN and 0.15 moles of KCN in water and making up die total volume to 0.5 dm3 is:

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Q. The dissociation constant of HCN is $ 5\times {{10}^{-8}} $ . The pH of the solution prepared by mixing 1.5 moles of HCN and 0.15 moles of KCN in water and making up die total volume to $ 0.5\,d{{m}^{3}} $ is:

JIPMERJIPMER 2001

Solution:

The solution of HCN and KCN is an acidic Buffer solution. Hence, from Hendarsons equation: $ pH=-\log \,{{K}_{a}}+\log \frac{[Salt]}{[Acid]} $ $ =-\log \,(5\times {{10}^{-8}})+\log \frac{0.15\text{/}0.5}{1.5\text{/}0.5} $ $ =-\log 5-\log {{10}^{-8}}+\log \frac{1}{10} $ $ =-\,0.6990+8+(-1) $ $ =6.3010 $