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