The following plot shows two pH titration curves, each representing the titration of 50.0 mL of 0.100 M acid with 0.100 M NaOH:

. (c) What is the approximate pKa of the weak acid?

Verified step by step guidance

1. Identify the titration curve corresponding to the weak acid. This is the curve that starts at a higher initial pH and has a more gradual slope before the equivalence point.
2. Locate the equivalence point on the titration curve of the weak acid. This is the point where the pH rapidly increases, indicating that the amount of NaOH added is stoichiometrically equivalent to the amount of weak acid present.
3. Determine the volume of NaOH added at the equivalence point. This is the volume at which the steepest part of the curve occurs.
4. Find the half-equivalence point, which is the point where half of the volume of NaOH needed to reach the equivalence point has been added. This is the midpoint of the buffer region on the titration curve.
5. Read the pH value at the half-equivalence point. The pH at this point is equal to the pKa of the weak acid.

Key Concepts

Titration Curves

Titration curves graphically represent the change in pH of a solution as a titrant is added. In this case, the curves show the titration of a strong base (NaOH) with a weak acid. The shape of the curve indicates the buffering region and the equivalence point, where the amount of acid equals the amount of base added.

pKa and Acid Strength

The pKa value is a measure of the strength of an acid in solution, defined as the negative logarithm of the acid dissociation constant (Ka). For weak acids, the pKa is typically found at the midpoint of the steepest part of the titration curve, where half of the acid has been neutralized, indicating the concentration of the acid equals that of its conjugate base.

Equivalence Point

The equivalence point in a titration is reached when the amount of titrant added is stoichiometrically equivalent to the amount of substance being titrated. For weak acids, this point is characterized by a rapid change in pH, and it is crucial for determining the pKa, as it indicates the complete neutralization of the acid by the base.