Relationship between Ka and Kb
If there is a solution of mix of a weak acid/base and it's alkaline/acidic salt in molar ratio 1:1, then


The solution is acidic, if $K_\mathrm{a} \gt K_\mathrm{b}$, like acetic acid + sodium acetate(alkaline base)
The solution is alkalic(ine)/basic, if $K_\mathrm{a} \lt K_\mathrm{b}$, like ammonium chloride ( acidic salt) + ammonia(base)


Note that $$\begin{align}
K_\mathrm{w}&amp;=\ce{[H+][OH-]} \\
K_\mathrm{a}&amp;=\frac{\ce{[H+][A-]}}
{\ce{[HA]}}  \\
K_\mathrm{b}&amp;=\frac{\ce{[HA][OH-]}}
{\ce{[A-]}}  \\
&amp;=\frac{\ce{[HA]  K_\mathrm{w}}}{\ce{[A-][H+]}}  \\
&amp;=\frac{\ce{  K_\mathrm{w}}}{K_\mathrm{a}}  \\
K_\mathrm{w}&amp;=K_\mathrm{a} \cdot K_\mathrm{b} \\
\end{align}$$

Similarly, if there is such a 1:1 (molar amount) solution of an acid and a base, that are not in the mutual relation acid/salt or base/salt, the above is valid as well.