Question

It has been found that the pH of a 0.01M solution of an organic acid is 4.15

Answer 1

Let the organic acid be HA.
⇒ HA $\leftrightarrow$ H+ +A-
Concentration of HA = 0.01 M
pH = 4.15
-log [H+] = 4.15 = [H+] = 7.08 × 10-5
Now, Ka = $\frac{[H^+][A^-]}{HA}$
[H+] = [A] = 7.08 × 10-5
[HA] = 0.01
Then, Ka = $\frac{(7.08 × 106{-5})(7.08 ×10^{-5})}{0.01}$
Ka = 5.01×10-7
pKa = -log Ka = -log(5.01 × 10-7)
pKa = 6.3001