The dissociation constant of acetic is $$x \times 10^{-5}$$. When 25 mL of 0.2 M CH$$_3$$COONa solution is mixed with 25 mL of 0.02 M CH$$_3$$COOH solution, the pH of the resultant solution is found to be equal to 5. The value of $$x$$ is _____.

After mixing: $$[CH_3COONa] = \frac{0.2 \times 25}{50} = 0.1$$ M, $$[CH_3COOH] = \frac{0.02 \times 25}{50} = 0.01$$ M.

Henderson-Hasselbalch: $$pH = pK_a + \log\frac{[salt]}{[acid]} = pK_a + \log\frac{0.1}{0.01} = pK_a + 1$$

$$5 = pK_a + 1 \Rightarrow pK_a = 4 \Rightarrow K_a = 10^{-4} = 10 \times 10^{-5}$$

$$x = 10$$.