Question

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.

Answer 1

Let the organic acid be HA
$\Rightarrow HA\leftrightarrow H^{+}+A^{-}$
Concentration of HA = 0.01 M pH
= 4.15
$-log\left[H^{+}\right]=4.15\left[H^{+}\right]=7.08\times {10}^{-5}$
$K_a=\frac{\left[H^{+}\right]\left[A^{-}\right]}{\left[HA\right]}$
Now,
$\left[H^{+}\right]=\left[A^{-}\right]=7.08\times {10}^{-5}\left[HA\right]=0.01$
Then,
$K_a=\frac{(7.08\times {10}^{-5})(7.08\times {10}^{-5})}{0.01}{K}_a=5.01\times {10}^{-7}p{K}_a=-log{K}_a=-log(5.01\times {10}^{-7})p{K}_a=6.3001$