# How do you determine the pKa of an acid?

We would use the relationship, $p H = p {K}_{a} + {\log}_{10} \left\{\frac{\left[{A}^{-}\right]}{\left[H A\right]}\right\}$
So the acid $H A$ would be titrated with small aliquots of base while the $p H$ of the solution was monitored with a $p H$ meter. At the point of half equivalence (which would only be apparent after the endpoint!), $\left[{A}^{-}\right] = \left[H A\right]$, and $p H = p {K}_{a}$ (because ${\log}_{10} 1 = 0$.