Ch.17 - Additional Aspects of Aqueous Equilibria

Problem 37b

Chapter 17, Problem 37b

Predict whether the equivalence point of each of the following titrations is below, above, or at pH 7: (b) NH₃ titrated with HCl.

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Identify the nature of the reactants: NH3 is a weak base and HCl is a strong acid.

Understand that the equivalence point in a titration occurs when the moles of acid equal the moles of base.

Recognize that at the equivalence point, the solution will primarily contain the conjugate acid of the weak base, NH4+.

Recall that NH4+ is a weak acid, which will slightly dissociate in water to produce H+ ions, lowering the pH.

Conclude that the equivalence point for the titration of NH3 with HCl will be below pH 7 due to the presence of NH4+.

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Key Concepts

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

Acid-Base Titration

An acid-base titration is a quantitative analytical method used to determine the concentration of an acid or base in a solution. During the titration, a solution of known concentration (the titrant) is added to a solution of unknown concentration until the reaction reaches its equivalence point, where the amount of acid equals the amount of base. The pH at this point can vary depending on the strengths of the acid and base involved.

Weak Base and Strong Acid

In the titration of a weak base, such as ammonia (NH3), with a strong acid like hydrochloric acid (HCl), the resulting solution at the equivalence point contains the conjugate acid (NH4+) of the weak base. This conjugate acid can donate protons to water, leading to a decrease in pH. Therefore, the pH at the equivalence point will be below 7, indicating an acidic solution.

pH Scale and Equivalence Point

The pH scale measures the acidity or basicity of a solution, with values below 7 indicating acidity, 7 being neutral, and above 7 indicating basicity. The equivalence point in a titration is critical for determining the pH of the resulting solution. For titrations involving a weak base and a strong acid, the pH at the equivalence point will typically be less than 7 due to the formation of a weakly basic conjugate acid.

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Related Practice

Textbook Question

The samples of nitric and acetic acids shown here are both titrated with a 0.100 M solution of NaOH(aq).

Determine whether each of the following statements concerning these titrations is true or false. (a) A larger volume of NaOH1aq2 is needed to reach the equivalence point in the titration of HNO3.

Textbook Question

Determine whether each of the following statements concerning the titrations in Problem 17.35 is true or false. b. Both titration curves will be essentially the same after passing the equivalence point.

Textbook Question

Predict whether the equivalence point of each of the following titrations is below, above, or at pH 7: (a) NaHCO₃ titrated with NaOH (c) KOH titrated with HBr.

Textbook Question

Predict whether the equivalence point of each of the following titrations is below, above, or at pH 7: (a) formic acid titrated with NaOH.

Textbook Question

Predict whether the equivalence point of each of the following titrations is below, above, or at pH 7: (b) calcium hydroxide titrated with perchloric acid.

Textbook Question

Predict whether the equivalence point of each of the following titrations is below, above, or at pH 7: (c) pyridine titrated with nitric acid.

Predict whether the equivalence point of each of the following titrations is below, above, or at pH 7: (b) NH3 titrated with HCl.

Verified video answer for a similar problem:

Key Concepts

Acid-Base Titration

Weak Base and Strong Acid

pH Scale and Equivalence Point

Predict whether the equivalence point of each of the following titrations is below, above, or at pH 7: (b) NH₃ titrated with HCl.