The dissociation constant of acetic acid is $x\times10^{-5}$. When 25 mL of 0.2 $\mathrm{M~CH_3COONa}$ solution is mixed with 25 mL of 0.02 $\mathrm{M~CH_3COOH}$ solution, the pH of the resultant solution is found to be equal to 5. The value of $x$ is ____________