Exp. 18: Pre-Lab

Question 1

Exp. 18: For a weak acid (e.g., CH3COOH) that is titrated with a strong base (e.g., NaOH), what species (ions/molecules) are present in the solution at the stoichiometric point?

Answer 1

At the stoichiometric point for a weak acid/strong base titration, the cation of the strong base (e.g. Na+) and the anion of the weak acid [the conjugate base of the weak acid (e.g. CH3CO2-)] are the predominant species in solution.

Question 2

Exp. 18: For a weak acid (e.g., CH3COOH) that is titrated with a strong base (e.g., NaOH), what species (ions/molecules) are present in the solution at the halfway point in the titration toward the stoichiometric point?

Answer 2

At the halfway point in the titration of a weak acid with a strong base, equal moles of weak acid, its conjugate base, and the cation of the strong base are present.

Question 3

Exp. 18: Briefly explain how the pKa for a weak acid is determined in this experiment.

Answer 3

The pKa of a weak acid is determined by titrating a sample of the acid to the stoichiometric point with
NaOH. A graph of [pH vs. volume NaOH dispensed] is then created and from this titration curve, the pH at the half-equivalence point is determined. At the half-equivalence point, the pH = pKa.