Question

For the titration of a weak acid with a strong base what is the pKa of the weak acid if the pH is 6.72 at the equilvalence point (100% T), 9.12 at the end point, and 4.23 at 50% T? Please answer with one sig fig.

Answer 1

Explanation:

We have to calculate $pK_{a}$ value.

It is known that at the equivalence point concentration of acid is equal to the concentration of anion formed.

Hence,         [HA] = $[A^{-}]$

Now, relation between $pK_{a}$ and pH is as follows.

                   pH = $pK_{a} + log \frac{[A^{-}]}{[HA]}$

Putting the values into the above formula as follows.

                   pH = $pK_{a} + log \frac{[A^{-}]}{[HA]}$

                  4.23 = $pK_{a} + log (1)$            (as [HA] = $[A^{-}]$)

                   $pK_{a}$ = 4.23                (as log (1) = 0)

or,                   $pK_{a}$ = 4

Thus, we can conclude that $pK_{a}$ of given weak acid is 4.