Question

I'm not sure if the premise of the question is even correct, but when adding ethanoic acid to sodium hydroxide solution, I was told that the pH at the point of neutralisation should be around 2x the pKa of ethanoic acid (so around 9.52), and the data I collected showed this as well. Why is this the case? I know that at the half equivalence point pH = pKa when adding a strong base to a weak acid, so what changed in this scenario?

Answer 1

The idea that the pH at the point of neutralisation is around twice the pKa of ethanoic acid is incorrect. The pH at the equivalence point during a titration of ethanoic acid with NaOH is above 7 due to the creation of the conjugate base, which makes the solution basic.

Step-by-step explanation:

The information provided about the pH at the point of neutralisation being around 2x the pKa of ethanoic acid is not accurate. During the titration of a weak acid like ethanoic acid with a strong base such as sodium hydroxide, the pH at the equivalence point is determined by the hydrolysis of the salt formed, not by the pKa value of the acid. As sodium hydroxide is added to ethanoic acid, the pH gradually increases.

However, at the equivalence point, the neutralisation is complete, and only a salt and water remain in solution. If the starting acid is weak and the base is strong, the salt solution is basic and the pH at the equivalence point is greater than 7. This is due to the presence of the conjugate base of the weak acid, which can accept protons from water, generating hydroxide ions and therefore an alkaline solution.