The dissociation constant of acetic acid is \( x \times 10^{-5} \). When \( 25 \, \mathrm{mL} \) of \( 0.2 \, \mathrm{M} \) \( \mathrm{CH_3COONa} \) solution is mixed with \( 25 \, \mathrm{mL} \) of \( 0.02 \, \mathrm{M} \) \( \mathrm{CH_3COOH} \) solution, the pH of the resultant solution is found to be equal to \( 5 \). The value of \( x \) is \( \_\_\_\_\_ \).
