Question

How are `pK_a`, and `K_a` values for weak acids found?

Answer 1

`pK_a` and `K_a` values are measured not found.

Explanation:

Weak acids, `HA`, dissociate according to the following equilibrium:

`HA(aq) + H_2O(l) rightleftharpoonsH_3O^+ + A^-`

By definition, `K_a=([H_3O^+][A^-])/([HA])`

We can take `-log_10` of both sides, to give............

`pH=pK_a + log_10([[A^-]]/[[HA]])`

Several glass electrodes are available that are calibrated to read `pH`. Both weak acids and weak bases can have their dissociation constants measured by these means. Of course, given the equation, the `pK_a` of the acid, is the `pH` at the point of half-equivalence. Why?