A buffer solution with a pH of $4\mathrm{.}0$ is made by dissolving Xg of potassium ethanoate $($CH$3$COOK$)$ in a solution of ethanoic acid $($CH$3$COOH$).$ The final volume of the buffer solution is $500$cm$3$ and the final concentration of the ethanoic acid is $0\mathrm{.}250$moldm$-3.$

What is the concentration of the H$+$ ions and of the CH$3$COO$-?$