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Ch.17 - Applications of Aqueous Equilibria
McMurry - Chemistry 8th Edition
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Capitolo 17, Problema 43

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.) Illustration of diprotic acid titration stages with varying pH levels.
. (b) Which solution has the highest pH? Which has the lowest pH?

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Identify the species present in each solution. The red spheres represent H2A, the purple spheres represent Na+, and the white spheres represent OH-.
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.
Recognize that the highest pH will be in the solution with the most OH- ions present, which is the second solution (top right).
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).
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).

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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.
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pH of Weak Diprotic Acids

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.
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The pH Scale

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.
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Equivalence Point in Titration
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The following pictures represent solutions of Ag2CrO4, which also may contain ions other than Ag+ and CrO42- that are not shown. Solution 1 is in equilibrium with solid Ag2CrO4. Will a precipitate of solid Ag2CrO4 form in solutions 2-4? Explain.

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The following pictures represent solutions at various stages in the titration of a weak base B with aqueous HCl. (Cl- ions and solvent water molecules have been omitted for clarity.)

. (b) Is the pH at the equivalence point more or less than 7?

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The following pictures represent solutions at various stages in the titration of sulfuric acid H2A (A2- = SO4 2-) with aqueous NaOH. (Na+ ions and water molecules have been omitted for clarity.)

. (b) Which solution has the highest pH? Draw a picture that represents the solution prior to addition of any NaOH.

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The following pictures represent solutions at various stages in the titration of a weak base B with aqueous HCl. (Cl- ions and solvent water molecules have been omitted for clarity.)

. (a) To which of the following stages do solutions 1–4 correspond? (i) The initial solution before addition of any HCl (ii) Halfway to the equivalence point (iii) At the equivalence point (iv) Beyond the equivalence point

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The following pictures represent solutions at various stages in the titration of sulfuric acid H2A (A2- = SO4 2-) with aqueous NaOH. (Na+ ions and water molecules have been omitted for clarity.)

. (a) To which of the following stages do solutions 1–4 correspond? (i) Halfway to the first equivalence point (ii) Halfway between the first and second equivalence points (iii) At the second equivalence point (iv) Beyond the second equivalence point

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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?

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