The following pictures represent solutions at various stages in the titration of a weak diprotic acid H2A with aqueous NaOH. (Na+ ions and water molecules have been omitted for clarity.)
. (b) Which solution has the highest pH? Which has the lowest pH?

Verified step by step guidance

1. Identify the species present in each solution. The red spheres represent H2A, the purple spheres represent Na+, and the white spheres represent OH-.
2. Determine the stage of titration for each solution. The first solution (top left) has H2A and Na+, indicating the initial stage. The second solution (top right) has Na+ and OH-, indicating the final stage.
3. Recognize that the highest pH will be in the solution with the most OH- ions present, which is the second solution (top right).
4. Identify the solution with the lowest pH, which will be the one with the most H2A present, indicating the initial stage of titration, which is the first solution (top left).
5. Conclude that the solution with the highest pH is the second solution (top right), and the solution with the lowest pH is the first solution (top left).

Key Concepts

Titration of Weak Diprotic Acids

Titration is a technique used to determine the concentration of a solution by reacting it with a solution of known concentration. In the case of weak diprotic acids, which can donate two protons (H+), the titration curve will show two distinct equivalence points, corresponding to the neutralization of each acidic proton. Understanding the stages of this titration is crucial for predicting pH changes throughout the process.

pH Scale

The pH scale measures the acidity or basicity of a solution, ranging from 0 (very acidic) to 14 (very basic), with 7 being neutral. The pH is logarithmically related to the concentration of hydrogen ions in the solution; thus, a small change in pH represents a significant change in hydrogen ion concentration. Knowing how to interpret pH values is essential for identifying which solution has the highest or lowest pH during titration.

Equivalence Points in Titration

In a titration, the equivalence point is reached when the amount of titrant added is stoichiometrically equivalent to the amount of substance in the solution being titrated. For weak diprotic acids, there are two equivalence points, each corresponding to the complete neutralization of one acidic proton. The pH at these points is typically higher than that of strong acid-strong base titrations, and understanding their locations on the titration curve is key to determining the pH of the solutions at various stages.