Quick questions on Weak acids and Ka: TCE Chemistry (Tasmania)

A weak acid $\text{HA}$ ionises only partly in water:

Set up an equilibrium with the initial acid concentration $c$. Let $x$ be the concentration of $\text{H}^+$ formed. Then $[\text{H}^+] = [\text{A}^-] = x$ and $[\text{HA}] = c - x$. Because the acid is weak, $x$ is small, so $c - x \approx c$.

Percentage ionisation $= \dfrac{[\text{H}^+]}{c} \times 100\%$. Diluting a weak acid increases the percentage ionised (Le Chatelier favours the side with more particles) even though $[\text{H}^+]$ falls. The approximation $c - x \approx c$ is valid when ionisation is below about $5\%$.

Because $K_a$ is fixed at a given temperature, it is the proper way to compare the intrinsic strength of acids, independent of concentration. pH, by contrast, depends on both strength and concentration, so two acids at the same pH need not be equally strong. When a question asks you to rank acids, use $K_a$ or $pK_a$: the larger the $K_a$ (or the smaller the $pK_a$), the stronger the acid and the more completely it ionises at a given concentration.