# How do you determine the Ka of a weak acid or the Kb of a weak base?

Apr 4, 2017

How else but by measurement?

#### Explanation:

Typically, a weak acid is titrated by a strong base, or a weak base is titrated by a strong acid. In order to measure ${K}_{a}$ (or more usually $- {\log}_{10} {K}_{a} = p {K}_{a}$).

We use the relationship:

$p H = p {K}_{a} + {\log}_{10} \left\{\frac{\left[{A}^{-}\right]}{\left[H A\right]}\right\}$

So do the measurement, i.e. the titration of weak acid by a strong base, and measure the $p H$ as you add increments of the titrant. At the point of HALF EQUIVALENCE, i.e. when $\left[{A}^{-}\right] = \left[H A\right]$, we use the given equation:

$p H = p {K}_{a} + {\log}_{10} \left\{1\right\} ,$ i.e. $p H = p {K}_{a}$, because ${\log}_{10} 1 = 0$.

Satisfied?