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Ch.17 - Applications of Aqueous Equilibria
Chapter 17, Problem 42b

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.)
Molecular representations of NH3, NH4+, and H3O+ during weak base titration with HCl.
. (b) Is the pH at the equivalence point more or less than 7?

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Titration

Titration is a quantitative analytical technique used to determine the concentration of a solute in a solution. It involves the gradual addition of a titrant (a solution of known concentration) to a sample solution until the reaction reaches its equivalence point, where the amount of titrant equals the amount of substance in the sample. In this case, the titration of a weak base with a strong acid (HCl) is being analyzed.
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Acid-Base Titration

Equivalence Point

The equivalence point in a titration is the stage at which the amount of titrant added is stoichiometrically equivalent to the amount of substance present in the sample. For the titration of a weak base with a strong acid, the equivalence point is characterized by the complete neutralization of the base, resulting in the formation of its conjugate acid and hydronium ions, which affects the pH of the solution.
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pH and Acid-Base Properties

pH is a measure of the acidity or basicity of a solution, with values below 7 indicating acidity and values above 7 indicating basicity. In the case of a weak base titrated with a strong acid, the pH at the equivalence point is typically less than 7 due to the presence of the conjugate acid formed during the reaction, which contributes to an increase in hydronium ion concentration.
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Related Practice
Textbook Question

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:

. (b) What is the approximate pH at the equivalence point for each of the acids?

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Textbook Question

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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Textbook Question

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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Textbook Question
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?
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Textbook Question

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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Textbook Question

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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