A buffer solution with a pH of $4\mathrm{.}0$ is made by dissolving X g 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 $0\mathrm{.}250$ mol dm$3.$ Calculate X in g for potassium ethanoate. Ka $=1\mathrm{.}74\backslash$times $105$ mol dm$3$