Ch.17 - Applications of Aqueous Equilibria

Problem 93a

Chapter 17, Problem 93a

What is the pH at the equivalence point for the titration of 0.10 M solutions of the following acids and bases, and which of the indicators in Figure 17.5 would be suitable for each titration? (a) HNO2 and NaOH

Verified step by step guidance

Step 1: Identify the type of acid and base involved in the titration. HNO2 is a weak acid and NaOH is a strong base.

Step 2: Determine the pH at the equivalence point for the titration of a weak acid with a strong base. At the equivalence point, the pH will be greater than 7 because the conjugate base of the weak acid (NO2-) will hydrolyze to form OH- ions.

Step 3: Calculate the concentration of the conjugate base (NO2-) at the equivalence point. Since the initial concentrations of HNO2 and NaOH are both 0.10 M, the concentration of NO2- at the equivalence point will also be 0.10 M.

Step 4: Use the Kb expression for the conjugate base (NO2-) to find the pH. The Kb can be found using the relationship Kb = Kw / Ka, where Kw is the ion-product constant for water (1.0 x 10^-14) and Ka is the acid dissociation constant for HNO2.

Step 5: Compare the calculated pH at the equivalence point to the pH ranges of the indicators in the table. Choose an indicator whose pH range includes the calculated pH at the equivalence point.

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Key Concepts

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

Equivalence Point in Titration

The equivalence point in a titration is the stage at which the amount of titrant added is stoichiometrically equivalent to the amount of substance being titrated. At this point, the reaction between the acid and base is complete, resulting in a neutralization reaction. The pH at the equivalence point can vary depending on the strength of the acid and base involved in the titration.

pH of Weak Acid and Strong Base Titration

In the titration of a weak acid, such as nitrous acid (HNO2), with a strong base like sodium hydroxide (NaOH), the pH at the equivalence point will be greater than 7. This is because the conjugate base formed from the weak acid hydrolyzes in water, producing hydroxide ions and resulting in a basic solution at the equivalence point.

Choosing Indicators for Titration

Selecting an appropriate indicator for a titration is crucial for accurately determining the endpoint. The indicator must change color at a pH that corresponds to the expected pH at the equivalence point. For the titration of HNO2 with NaOH, an indicator that changes color around a pH greater than 7, such as Thymolphthalein, would be suitable.

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Related Practice

Textbook Question

On the same graph, sketch pH titration curves for the titra-tion of (1) a strong acid with a strong base and (2) a weak acid with a strong base. How do the two curves differ with respect to the following? (a) The initial pH

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Textbook Question

The equivalence point was reached in titrations of three unknown acids at pH 9.16 (acid A), 8.88 (acid B), and 8.19 (acid C). (a) Which is the strongest acid?(b) Which is the weakest acid?

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Textbook Question

What is the pH at the equivalence point for the titration of 0.10 M solutions of the following acids and bases, and which of the indicators in Figure 17.5 would be suitable for each titration? (c) CH3NH2 (methylamine) and HCl

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Textbook Question

What is the pH at the equivalence point for the titration of 0.20 M solutions of the following acids and bases? Which of the indicators in Figure 17.5 would be suitable for each titration? (c) Ba(OH)2 and HBr

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Textbook Question

Consider the titration of 50.0 mL of a 0.100 M solution of the protonated form of the amino acid alanine (H₂A⁺: K_a1 = 4.6 × 10^–3, K_a2 = 2.0 × 10^–10) with 0.100 M NaOH. Calculate the pH after the addition of each of the following volumes of base. (a) 10.0 mL

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