The pH of a weak acid

You will write the functions that turn a pKa and a concentration into the pH. Then you will find how much of the acid is ionised.

The idea. $K_a = 10^{-\text{p}K_a}$. For $c$ mol/L of acid, $x = [\text{H}^+]$ is the positive root of $x^2 + K_a x - K_a c = 0$.

Your tasks.

1. Complete ka_from_pka(pka).
2. Complete hydrogen_ion(pka, c) using the quadratic formula.
3. Complete percent_ionised(pka, c).