# If given a pH of 2.57, how do you find the pKa?

You can find $\left[{H}_{3} {O}^{+}\right]$, but you cannot find $p {K}_{a}$ with the given information.
If $p H$ $=$ $2.57$, then (by definition), $\left[{H}_{3} {O}^{+}\right]$ $=$ ${10}^{- 2.57}$ $m o l \cdot {L}^{-} 1$. We have no handle on the concentration of the acid that gave rise to this concentration of hydronium ion.