Problem 119 - Chemistry: An Atoms-Focused Approach

Question

For each set of three acids, which one should you use to make a buffer with the given pH? a. Select from acetic acid \(\left(\mathrm{p} K_{2} 4.75\right),\) fluoroacetic acid \(\left(\mathrm{p} K_{\mathrm{a}} 2.59\right)\) and hypochlorous acid \(\left(\mathrm{p} K_{\mathrm{a}} 7.54\right)\) to make a buffer with a pH of 5.2. b. Select from formic acid \(\left(\mathrm{p} K_{2}=3.75\right),\) hypobromous acid \(\left(\mathrm{p} K_{\mathrm{a}}=7.54\right),\) and boric acid \(\left(\mathrm{p} K_{2} 9.27\right)\) to make a buffer with a pH of 8.0.

Step-by-Step Solution

Answer

Answer: Acetic acid is the most suitable acid to use as a buffer for a pH of 5.2, and hypobromous acid is the most suitable acid to use as a buffer for a pH of 8.0.

Step 1: List the acids and their pKa values

We are given the following acids and their pKa values: - Acetic acid: pKa = 4.75 - Fluoroacetic acid: pKa = 2.59 - Hypochlorous acid: pKa = 7.54

Step 2: Compare pH value to pKa values

To find the acid that makes the best buffer at pH 5.2, we compare this pH value to the pKa values of the given acids: - Acetic acid: |5.2 - 4.75| = 0.45 - Fluoroacetic acid: |5.2 - 2.59| = 2.61 - Hypochlorous acid: |5.2 - 7.54| = 2.34

Step 3: Select the acid with the closest pKa value

Acetic acid has the smallest difference between the pKa value and the desired pH (0.45), so it is the best choice for a buffer at pH 5.2. b. Which acid to use for a buffer with a pH of 8.0?

Step 1: List the acids and their pKa values

We are given the following acids and their pKa values: - Formic acid: pKa = 3.75 - Hypobromous acid: pKa = 7.54 - Boric acid: pKa = 9.27

Step 2: Compare pH value to pKa values

To find the acid that makes the best buffer at pH 8.0, we compare this pH value to the pKa values of the given acids: - Formic acid: |8.0 - 3.75| = 4.25 - Hypobromous acid: |8.0 - 7.54| = 0.46 - Boric acid: |8.0 - 9.27| = 1.27

Step 3: Select the acid with the closest pKa value

Hypobromous acid has the smallest difference between the pKa value and the desired pH (0.46), so it is the best choice for a buffer at pH 8.0.

Key Concepts

Understanding pKa Values

The pKa value of an acid is a critical concept in chemistry. It tells us how easily an acid donates its proton, which is essential for predicting the acidity of solutions.
Lower pKa values indicate stronger acids that are more willing to release their protons, while higher pKa values correspond to weaker acids that hold onto their protons more tightly.
Understanding pKa values is crucial when creating buffer solutions. Buffers are needed to maintain a stable pH, and the right buffer depends on choosing an acid with a pKa close to the target pH of the solution.

• This is because, at a pH equal to the pKa, the acid and its conjugate base are present in equal amounts, providing maximum buffering capacity.
• Any deviation from the pH close to the pKa reduces the buffer's ability to resist changes in pH when acids or bases are added.

Thus, the essence of pKa lies in its ability to guide the selection of appropriate acids for buffers, ensuring solutions retain their desired acidity levels.

Choices in Acid Selection

Selecting an acid for a buffer solution involves more than just comparing pKa values. It requires understanding the target pH and how the buffer will function under specific conditions.
Choosing an acid with a pKa close to your desired pH is vital for effective buffering. This proximity allows the buffer solution to resist pH changes more effectively.
For instance, if your target pH is 5.2, like in the exercise with acetic acid, fluoroacetic acid, and hypochlorous acid, you would select an acid whose pKa minimizes the difference with 5.2.

• Check the absolute difference between the pKa of each acid and the desired pH.
• The acid with the smallest difference will generally make for the most suitable buffer.

By carefully assessing the pKa values and desired pH, you can make informed decisions that lead to more stable and effective buffer systems.

Applying the Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation is a fundamental tool in buffer chemistry, allowing us to calculate pH or predict how additions of acid or base will affect the buffer's pH.This equation is written as: \[pH = pK_a + \log{\left(\frac{[A^-]}{[HA]}\right)}\]where - \([A^-]\) is the concentration of the base form,- \([HA]\) is the concentration of the acid form.To apply this equation for buffers, ensure your chosen acid and base are in equilibrium, ideally when the pKa is near your desired pH.
A well-chosen buffer will maintain its pH even if small amounts of acids or bases are added, due to the balanced ratio of \([A^-]\) and \([HA]\) at equilibrium.
Here are steps to effectively use the equation:

• Start by selecting an acid with a pKa that matches your target pH.
• Adjust the concentrations of the acid and conjugate base to match the desired pH.
• Ensure the buffer capacity is adequate by keeping enough buffering components.

Mastering this equation is a pivotal skill in chemistry, providing the framework to design buffers that maintain a consistent environment, crucial for myriad chemical and biological applications.