The dissociation constant of a weak acid is $1\times {10}^{-4}$. In order to prepare a buffer solution with a pH=5, the [Salt]/[Acid] ratio should be

$$\begin{gathered} \mathrm{pH}={\mathrm{pK}}_{\mathrm{a}}+\log \frac{\left[\text{ Salt }\right]}{\left[\mathrm{Acid}\right]} \\ 5=-{\mathrm{logK}}_{\mathrm{a}}+\log \frac{\left[\text{ Salt }\right]}{\left[\text{ Acid }\right]} \left[\because {\mathrm{pK}}_{\mathrm{a}}=-{\mathrm{logK}}_{\mathrm{a}}\right] \\ 5=-\log \left[1\times {10}^{-4}\right]+\log \frac{\left[\text{ Salt }\right]}{\left[\text{ Acid }\right]} \\ 5=4+\log \frac{\left[\text{ Salt }\right]}{\left[\text{ Acid }\right]} \\ 5-4=\log \frac{\left[\text{ Salt }\right]}{\left[\text{ Acid }\right]} \\ 1=\log \frac{\left[\text{ Salt }\right]}{\left[\text{ Acid }\right]} \\ \frac{\left[\text{ Salt }\right]}{\text{ [Acid] }}=10=10:1 \end{gathered}$$