Ch.17 - Applications of Aqueous Equilibria

McMurry8th EditionChemistryISBN: 9781292336145Not the one you use?Change textbook

McMurry 8th Edition

Ch.17 - Applications of Aqueous Equilibria

Problem 42a

Chapter 17, Problem 42a

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

Verified step by step guidance

Identify the species present in each solution: The blue spheres represent the weak base B, the red spheres represent H+ ions from HCl, and the white spheres represent the conjugate acid BH+.

Analyze solution 1: It contains only blue spheres, indicating it is the initial solution before any HCl is added.

Analyze solution 2: It contains both blue and white spheres, indicating the presence of both the weak base B and its conjugate acid BH+. This corresponds to the stage halfway to the equivalence point.

Analyze solution 3: It contains only white spheres, indicating that all the weak base B has been converted to its conjugate acid BH+. This corresponds to the equivalence point.

Analyze solution 4: It contains white spheres and red spheres, indicating the presence of excess H+ ions beyond the equivalence point.

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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 until a reaction reaches its endpoint, which is often indicated by a color change or a pH change. In this case, the titration of a weak base with a strong acid illustrates how the pH of the solution changes at different stages of the reaction.

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 a weak base titrated with a strong acid, this point is characterized by a significant change in pH, typically occurring when all the weak base has reacted with the acid. Understanding this concept is crucial for identifying the correct stage in the titration process.

Weak Base and Strong Acid Behavior

Weak bases, such as ammonia, do not completely dissociate in solution, resulting in a higher pH compared to strong bases. When titrated with a strong acid like HCl, the weak base will react to form its conjugate acid, leading to a gradual decrease in pH until the equivalence point is reached. Beyond this point, the solution becomes increasingly acidic, and understanding this behavior is essential for interpreting the stages of the titration represented in the images.

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Strong Acid-Strong Base Titration

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 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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Textbook Question

The following pictures represent solutions that contain one or more of the compounds H2A, NaHA, and Na2A, where H2A is a weak diprotic acid. (Na+ ions and solvent water molecules have been omitted for clarity.)

(b) Which solution has the greatest buffer capacity?

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Textbook Question

(a) Which of the solutions are buffer solutions?

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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.)

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

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