A buffer solution with a pH of $4\mathrm{.}0$ is made by dissolving $xg$ of potassium ethanoate $(C{H}_3\frac{C}{O}OK)$ in a solution of ethanoic acid $(C{H}_3\frac{C}{O}OH).$ The final volume of the buffer solution is $500c{m}^3$ and the final concentration of the ethanoic acid is $0\mathrm{.}250$moldm. What is the concentration of CH$3$COO$-$