Ch.16 - Aqueous Equilibria: Acids & Bases

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

McMurry 8th Edition

Ch.16 - Aqueous Equilibria: Acids & Bases

Problem 17-148e

Chapter 16, Problem 17-148e

A 40.0 mL sample of a mixture of HCl and H3PO4 was titrated with 0.100 M NaOH. The first equivalence point was reached after 88.0 mL of base, and the second equivalence point was reached after 126.4 mL of base.
(e) Sketch the pH titration curve, and label the buffer regions and equivalence points.

Verified step by step guidance

Identify the acids in the mixture: HCl is a strong acid, and H3PO4 is a triprotic acid with three dissociation steps.

Understand that the first equivalence point corresponds to the neutralization of HCl and the first dissociation of H3PO4.

Recognize that the second equivalence point corresponds to the neutralization of the second dissociation of H3PO4.

Sketch the titration curve with volume of NaOH on the x-axis and pH on the y-axis. Start with a low pH due to the presence of strong acid HCl.

Label the first equivalence point at 88.0 mL and the second equivalence point at 126.4 mL. Indicate buffer regions between these points where the pH changes more gradually.

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Key Concepts

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

Titration and Equivalence Points

Titration is a quantitative analytical method used to determine the concentration of a solute in a solution. The equivalence point in a titration occurs when the amount of titrant added is stoichiometrically equivalent to the amount of substance in the sample. In this case, the first and second equivalence points indicate the complete neutralization of HCl and H3PO4, respectively, by NaOH.

pH and Buffer Regions

pH is a measure of the acidity or basicity of a solution, with lower values indicating acidic conditions and higher values indicating basic conditions. Buffer regions in a titration curve are areas where the pH changes gradually, typically occurring before and after the equivalence points. These regions are important for understanding how the solution resists changes in pH due to the presence of weak acids or bases.

Titration Curves for Polyprotic Acids

Polyprotic acids, like H3PO4, can donate more than one proton, leading to multiple equivalence points in a titration curve. Each equivalence point corresponds to the complete neutralization of one acidic proton. The shape of the titration curve reflects these transitions, showing steep rises at equivalence points and flatter regions in between, which are critical for identifying the behavior of the acid during titration.

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Acid-Base Titration Curves

Related Practice

Textbook Question

A 100.0 mL sample of a solution that is 0.100 M in HCl and 0.100 M in HCN is titrated with 0.100 M NaOH. Calculate the pH after the addition of the following volumes of NaOH:

(a) 0.0 mL

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Textbook Question

Locate sulfur, selenium, chlorine, and bromine in the periodictable:(a) Which binary acid 1H2S, H2Se, HCl, or HBr2 is the strongest?Which is the weakest? Explain.

277

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Textbook Question

What is the pH and the principal source of H3O+ ions in 1.0 * 10-10 M HCl? (Hint: The pH of an acid solution can’t exceed 7.) What is the pH of 1.0 * 10-7 M HCl?

107

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Textbook Question

The following pictures represent aqueous solutions of three acids HA1A = X, Y, or Z2; water molecules have been omitted for clarity.

(e) What is the percent dissociation in the solution of HZ?

487

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Textbook Question

The following pictures represent aqueous solutions of three acids HA1A = X, Y, or Z2; water molecules have been omitted for clarity.

814

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A 40.0 mL sample of a mixture of HCl and H3PO4 was titrated with 0.100 M NaOH. The first equivalence point was reached after 88.0 mL of base, and the second equivalence point was reached after 126.4 mL of base. (e) Sketch the pH titration curve, and label the buffer regions and equivalence points.

Verified video answer for a similar problem:

Key Concepts

Titration and Equivalence Points

pH and Buffer Regions

Titration Curves for Polyprotic Acids

A 40.0 mL sample of a mixture of HCl and H3PO4 was titrated with 0.100 M NaOH. The first equivalence point was reached after 88.0 mL of base, and the second equivalence point was reached after 126.4 mL of base.
(e) Sketch the pH titration curve, and label the buffer regions and equivalence points.