Ch.17 - Additional Aspects of Aqueous Equilibria

Problem 20

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Chapter 17, Problem 20

Which of the following solutions is a buffer? (a) A solution made by mixing 100 mL of 0.100 M CH3COOH and 50 mL of 0.100 M NaOH, (b) a solution made by mixing 100 mL of 0.100 M CH3COOH and 500 mL of 0.100 M NaOH, (c) A solution made by mixing 100 mL of 0.100 M CH3COOH and 50 mL of 0.100 M HCl, (d) A solution made by mixing 100 mL of 0.100 M CH3COOK and 50 mL of 0.100 M KCl.

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Identify the components needed for a buffer solution: a weak acid and its conjugate base, or a weak base and its conjugate acid.

Analyze option (a): Calculate the moles of CH3COOH and NaOH. Determine if there is enough NaOH to partially neutralize CH3COOH, forming CH3COO- (acetate), the conjugate base.

Analyze option (b): Calculate the moles of CH3COOH and NaOH. Check if NaOH completely neutralizes CH3COOH, leaving no weak acid to form a buffer.

Analyze option (c): Calculate the moles of CH3COOH and HCl. Since HCl is a strong acid, it will not form a buffer with CH3COOH.

Analyze option (d): Check if CH3COOK (potassium acetate) and KCl can form a buffer. Note that KCl is a neutral salt and does not contribute to buffer formation.

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Key Concepts

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

Buffer Solutions

A buffer solution is a system that resists changes in pH upon the addition of small amounts of acid or base. It typically consists of a weak acid and its conjugate base or a weak base and its conjugate acid. This equilibrium allows the buffer to neutralize added acids or bases, maintaining a relatively stable pH.

Weak Acids and Conjugate Bases

Weak acids, such as acetic acid (CH3COOH), partially dissociate in solution, establishing an equilibrium between the undissociated acid and its ions. The conjugate base, in this case, acetate (CH3COO-), can react with added hydrogen ions (H+) to minimize pH changes, making the combination of a weak acid and its conjugate base essential for buffer action.

Stoichiometry in Buffer Preparation

Stoichiometry involves the calculation of reactants and products in chemical reactions. In the context of buffer solutions, it is crucial to determine the ratios of weak acid to conjugate base. The effectiveness of a buffer depends on the concentrations of these components, which can be influenced by the volumes and molarities of the solutions mixed.

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Buffer Capacity

Related Practice

a. Calculate the percent ionization of 0.007 M butanoic acid (𝐾𝑎=1.5×10−5).

Textbook Question

(b) Calculate the percent ionization of 0.0075 M butanoic acid in a solution containing 0.085 M sodium butanoate.

(a) Calculate the percent ionization of 0.125 M lactic acid 1Ka = 1.4 * 10-42.

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Textbook Question

(a) Calculate the pH of a buffer that is 0.12 M in lactic acid and 0.11 M in sodium lactate.

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Textbook Question

You are asked to prepare a pH = 3.00 buffer solution starting from 1.25 L of a 1.00 M solution of hydrofluoric acid (HF) and any amount you need of sodium fluoride (NaF). (a) What is the pH of the hydrofluoric acid solution prior to adding sodium fluoride?

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Textbook Question

You are asked to prepare a pH = 3.00 buffer solution starting from 1.25 L of a 1.00 M solution of hydrofluoric acid (HF) and any amount you need of sodium fluoride (NaF). (b) How many grams of sodium fluoride should be added to prepare the buffer solution? Neglect the small volume change that occurs when the sodium fluoride is added.

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