Ncert Exercises & Solutions

7.47 It has been found that the pH of a 0.01M solution of an organic acid is 4.15. Calculate the concentration of the anion, the ionization constant of the acid and its pKa.

Let the organic acid be HA . \Rightarrow HA \leftrightarrow H^{+} + A^{-} Concentration of HA = 0.01 M pH = 4.15 - \log [H^{+}] = 4.15 [H^{+}] = 7.08 \times 10^{- 5} K_{a} = \frac{[H^{+}] [A^{-}]}{[HA]} Now, [H^{+}] = [A^{-}] = 7.08 \times 10^{- 5} [HA] = 0.01 Then, K_{a} = \frac{(7.08 \times 10^{- 5}) (7.08 \times 10^{- 5})}{0.01} K_{a} = 5.01 \times 10^{- 7} {pK}_{a} = \log K_{a} = - \log (5.01 \times 10^{- 7}) {pK}_{a} = 6.3001

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