Chemistry 102: Acid Equilibrium: Write estimated pKa

Question

4. The graph shows the titration curves of a strong acid and a weak acid. Estimate the pK_a of the weak acid from its titration curve.

 

Write the estimated pK_a to one decimal place.

Answer 1

The titration curve for the WEAK acid would be the line shown in RED. We know this because it is much less steep than the other curve which would be the curve for the strong acid. We can estimate the pKa of the weak acid by finding the pH at the 1/2 equivalence point, which would be slightly above pH = 6

The pKa would thus be ~ 6.2

When adding OH- to a weak acid, you create a buffer and when the [conjugate base] = [weak acid], the

pH = pKa. This can be seen in the Henderson Hasselbalch equation: pH = pKa + log [conj.base] / [acid].