The following figure represents solutions at various stages of the titration of a weak acid, HA, with NaOH. (The Na+ ions and water molecules have been omitted for clarity.) To which of the following regions of the titration curve does each drawing correspond: (b) after addition of NaOH but before the equivalence point? [Section 17.3]

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Identify the species present in each stage of the titration: HA (weak acid), A^- (conjugate base), and OH^- (hydroxide ion).

Understand that before the equivalence point, the solution contains both the weak acid (HA) and its conjugate base (A^-), but not significant amounts of OH^-.

Analyze the images: (a) shows only HA, (b) shows both HA and A^-, (c) shows A^-, HA, and OH^-, and (d) shows mostly HA and A^- with some OH^-.

Determine that image (b) corresponds to the stage after the addition of NaOH but before the equivalence point, as it shows both HA and A^- without significant OH^- presence.

Conclude that image (b) represents the solution after the addition of NaOH but before the equivalence point in the titration curve.

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Key Concepts

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

Titration Curve

A titration curve is a graphical representation of the pH of a solution as a function of the volume of titrant added. In the case of a weak acid being titrated with a strong base, the curve typically shows a gradual increase in pH, with a steep rise near the equivalence point. Understanding the shape of the curve helps in identifying different stages of the titration process, including the region before the equivalence point.

Weak Acid and Conjugate Base

A weak acid, such as HA, partially dissociates in solution to form its conjugate base (A-) and hydrogen ions (H+). During titration with a strong base like NaOH, the weak acid reacts with hydroxide ions (OH-) to form water and its conjugate base. This equilibrium is crucial for understanding the composition of the solution at various points in the titration, particularly before reaching the equivalence point.

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 solution. For a weak acid titrated with a strong base, this point is characterized by a significant change in pH and the complete conversion of the weak acid to its conjugate base. Recognizing the region before this point is essential for analyzing the titration curve and the solution's composition.

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At the Equivalence Point

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Textbook Question

A buffer contains a weak acid, HA, and its conjugate base. The weak acid has a pKa of 4.5, and the buffer has a pH of 4.3. Without doing a calculation, state which of these possibilities are correct at pH 4.3. (a) 3HA4 = 3A-4, (b) 3HA4 7 3A-4, or (c) 3HA4 6 3A-4. [Section 17.2]

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Textbook Question

The following diagram represents a buffer composed of equal concentrations of a weak acid, HA, and its conjugate base, A-. The heights of the columns are proportional to the concentrations of the components of the buffer. (a) Which of the three drawings, (1), (2), or (3), represents the buffer after the addition of a strong acid? [Section 17.2]

Textbook Question

The following diagram represents a buffer composed of equal concentrations of a weak acid, HA, and its conjugate base, A-. The heights of the columns are proportional to the concentrations of the components of the buffer. (c) Which of the three represents a situation that cannot arise from the addition of either an acid or a base? [Section 17.2]

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Textbook Question

Match the following descriptions of titration curves with the diagrams: (a) strong acid added to strong base. [Section 17.3]

Textbook Question

Match the following descriptions of titration curves with the diagrams: (d) strong base added to polyprotic acid. [Section 17.3]