Problem 19 - Chemistry: The Central Science in SI Units, Global Edition

Question

Which of the following solutions is a buffer? (a) \(0.20 \mathrm{M}\) for\(\operatorname{mic}\) acid \((\mathrm{HCOOH}),(\mathbf{b}) 0.20 M\) formic acid \((\mathrm{HCOOH})\) and \(0.20 \mathrm{M}\) sodium formate \((\mathrm{HCOONa}),(\mathbf{c}) 0.20 \mathrm{Mnitric}\) acid \(\left(\mathrm{HNO}_{3}\right)\) and \(0.20 \mathrm{M}\) sodium nitrate \(\left(\mathrm{NaNO}_{3}\right)\) (d) both b and \(\mathrm{c},(\mathbf{e})\) all of \(\mathrm{a}, \mathrm{b},\) and \(\mathrm{c}\).

Step-by-Step Solution

Answer

The correct answer is (b) \(0.20 \mathrm{M}\) formic acid \((\mathrm{HCOOH})\) and \(0.20 \mathrm{M}\) sodium formate \((\mathrm{HCOONa})\), as this is the buffer solution among the given options.

Step 1: Identify weak acids and their conjugate bases in the given solutions

In this step, we will identify the weak acids and their conjugate bases (or weak bases and their conjugate acids) in each given solution: (a) \(0.20 \mathrm{M}\) formic acid \((\mathrm{HCOOH})\): This solution has only a weak acid, and its conjugate base is not present. (b) \(0.20 \mathrm{M}\) formic acid \((\mathrm{HCOOH})\) and \(0.20 \mathrm{M}\) sodium formate \((\mathrm{HCOONa})\): This solution has both a weak acid and its conjugate base present. (c) \(0.20 \mathrm{M}\) nitric acid \(\left(\mathrm{HNO}_{3}\right)\) and \(0.20 \mathrm{M}\) sodium nitrate \(\left(\mathrm{NaNO}_{3}\right)\): Nitric acid (\(\mathrm{HNO}_{3}\)) is a strong acid, so this solution does not contain a weak acid and its conjugate base.

Step 2: Identify the buffer solution(s)

Based on our analysis in Step 1, we can now identify the buffer solution(s) from the given options: (a) \(0.20 \mathrm{M}\) formic acid \((\mathrm{HCOOH})\): Not a buffer solution. (b) \(0.20 \mathrm{M}\) formic acid \((\mathrm{HCOOH})\) and \(0.20 \mathrm{M}\) sodium formate \((\mathrm{HCOONa})\): This is a buffer solution because it contains both a weak acid and its conjugate base. (c) \(0.20 \mathrm{M}\) nitric acid \(\left(\mathrm{HNO}_{3}\right)\) and \(0.20 \mathrm{M}\) sodium nitrate \(\left(\mathrm{NaNO}_{3}\right)\): Not a buffer solution. (d) Both b and c: Not valid since option (c) is not a buffer solution. (e) All of a, b, and c: Not valid since options (a) and (c) are not buffer solutions. From our evaluation, we can conclude that the correct answer is (b) \(0.20 \mathrm{M}\) formic acid \((\mathrm{HCOOH})\) and \(0.20 \mathrm{M}\) sodium formate \((\mathrm{HCOONa})\), as this is the buffer solution among the given options.

Key Concepts

Weak Acids

Weak acids are crucial components in chemistry and buffer solutions. Unlike strong acids, which completely dissociate into their ions in water, weak acids only partially dissociate. This means not all of the acid molecules break apart into ions. This partial dissociation establishes an equilibrium between the undissociated acid and its ions.
For example:

• Acetic acid: A common weak acid known for its role in vinegar.
• Formic acid (HCOOH): Used in the food industry and as a buffer component.

A weak acid's power comes from its ability to release fewer hydrogen ions (\(H^+\)), hence it doesn't decrease pH as drastically as strong acids. This property is essential in buffer solutions, where controlling changes in pH is necessary.

Conjugate Bases

Conjugate bases are the partners to weak acids in buffer solutions. When a weak acid loses a hydrogen ion, it forms its conjugate base. This base has the ability to re-associate with hydrogen ions, contributing to the stability of a solution's pH.
For instance, when formic acid (\( ext{HCOOH} \)) donates a proton, its conjugate base is the formate ion (\( ext{HCOO}^- \)).

• Function: Conjugate bases capture and neutralize additional hydrogen ions, helping to maintain equilibrium.
• Buffer Role: They work alongside weak acids to mitigate pH changes.

In a buffer, both the weak acid and its conjugate base are present in significant amounts. This setup allows the solution to resist pH shifts when small amounts of acid or base are added.

Formic Acid

Formic acid (\( ext{HCOOH} \)), the simplest carboxylic acid, plays a vital role in buffer solutions due to its weak acidic properties. It's used in many industrial applications, such as leather tanning and food preservation. In a chemical context, formic acid partly dissociates in water:\[ ext{HCOOH} ightleftharpoons ext{H}^+ + ext{HCOO}^- \]

• Dissociation: Only partially in water, allowing equilibrium with its conjugate base.
• Application: Commonly used in labs for preparing buffer solutions.

When dissolved, formic acid provides both hydrogen ions (\( ext{H}^+ \)) and formate ions (\( ext{HCOO}^- \)), perfect for establishing a pH buffer system.

Sodium Formate

Sodium formate (\( ext{HCOONa} \)) is the sodium salt of formic acid. In aqueous solutions, it dissociates completely into sodium ions (\( ext{Na}^+ \)) and formate ions (\( ext{HCOO}^- \)). This dissociation makes sodium formate an essential component in buffer systems.

• Conjugate Base Role: Provides formate ions, the conjugate base of formic acid, ready to react with additional hydrogen ions.
• Buffer Creation: When combined with formic acid, it forms a buffer solution capable of resisting pH changes.

By including both formic acid and sodium formate, the solution can maintain a stable pH, even if small amounts of acids or bases are introduced. This quality is what qualifies it as an effective buffer solution.