Question

You have the following reagents on hand: $$ \begin{array}{ll} \text { Solids }\left(\mathrm{pK}_{\mathrm{a}}\right. \text { of Acid } \\ \text { Form Is Given) } & \text { Solutions } \\ \hline \text { Benzoic acid (4.19) } & 5.0 \mathrm{M} \mathrm{HCl} \\ \text { Sodium acetate (4.74) } & 1.0 \mathrm{M} \text { acetic acid (4.74) } \\\ \text { Potassium fluoride (3.14) } & 2.6 \mathrm{M} \mathrm{NaOH} \\ \text { Ammonium chloride (9.26) } & 1.0 \mathrm{M} \text { HOCl }(7.46) \\ \hline \end{array} $$ What combinations of reagents would you use to prepare buffers at the following \(\mathrm{pH}\) values? a. \(3.0\) b. \(4.0\) c. \(5.0\) d. \(7.0\) e. \(9.0\)

Short answer

a. Potassium fluoride & 2.6 M NaOH b. Benzoic acid & Sodium acetate c. Benzoic acid & 5.0 M HCl d. 1.0 M acetic acid & 1.0 M HOCl e. Ammonium chloride & 2.6 M NaOH

Step-by-step solution

Find combinations for pH 3.0

For pH 3.0, we'll look for pairs with a difference in pKa value around 1. Potassium fluoride with a pKa value of 3.14 is suitable in this case when combined with its acidic counterpart, 2.6 M NaOH.

Find combinations for pH 4.0

For pH 4.0, we can use Benzoic acid with a pKa value of 4.19 and Sodium acetate with a pKa value of 4.74. The difference in pKa values is close to zero, making it an ideal pair for forming a buffer.

Find combinations for pH 5.0

For pH 5.0, Benzoic acid with a pKa value of 4.19 and 5.0 M HCl can be used. The difference in pKa values is close to 1, making it a suitable pair for forming a buffer.

Find combinations for pH 7.0

For pH 7.0, we can use 1.0 M acetic acid with a pKa value of 4.74 and 1.0 M HOCl with a pKa value of 7.46. The difference in pKa values is close to 1, making it an appropriate pair for forming a buffer.

Find combinations for pH 9.0

For pH 9.0, we can use Ammonium chloride with a pKa value of 9.26. Combine it with 2.6 M NaOH, a basic counterpart. The difference in pKa values is close to 1, making it an optimal pair for forming a buffer. In conclusion, the reagents' combinations for preparing buffers with the desired pH values are: a. Potassium fluoride & 2.6 M NaOH b. Benzoic acid & Sodium acetate c. Benzoic acid & 5.0 M HCl d. 1.0 M acetic acid & 1.0 M HOCl e. Ammonium chloride & 2.6 M NaOH

Key concepts

pKa value

Understanding the pKa value is essential in chemistry, particularly when preparing buffer solutions. The pKa value is a numerical scale used to measure the acidity or basicity of a substance. It represents the equilibrium constant for the dissociation of a weak acid into its conjugate base and a hydrogen ion. In simpler terms, it tells us how easily an acid can donate a proton. The lower the pKa value, the stronger the acid, and the higher the pKa value, the weaker the acid.

For preparing buffers, which are solutions that resist changes in pH when acids or bases are added, the pKa value is crucial. The best buffers work near their pKa values, generally within one pH unit. This is because a buffer made from a weak acid and its conjugate base (or weak base and its conjugate acid) has its maximum buffering capacity at a pH near the pKa of the acid (or base). Thus, selecting an acid and base pair with pKa values close to the desired buffer pH is important for effective buffering.

Importance in Buffer Preparation

In the exercise provided, the pKa values are given for various acidic and basic solids, which helps in determining which combinations would make a suitable buffer for a specific pH level. For instance, a buffer for pH 3.0 was formed using potassium fluoride with a pKa value of 3.14, matching the required pH. This shows that a fundamental understanding of pKa values directs the buffer preparation process.

pH level

The pH level is a measure of the hydrogen ion concentration in a solution and reflects how acidic or alkaline that solution is. The pH scale ranges from 0 to 14 with 0 being very acidic, 7 being neutral, and 14 being very alkaline. Each unit change in the pH scale corresponds to a tenfold change in hydrogen ion concentration.

For a chemist, the pH level is a key parameter to monitor and control, especially when creating buffer solutions. A buffer solution is designed to maintain a stable pH even when small amounts of acid or base are added. This property is critical in many biological and chemical processes where a stable pH is required.

Buffer Solution Application

When deciding on a buffer, it is essential to look for a system that can maintain the needed pH level. As seen in the exercise example, to create buffers at various pH levels, compounds with pKa values close to the target pH were selected. This precision in choosing appropriate pKa values helps to create a buffer that effectively maintains the desired pH, as it will be most resilient to pH changes in that range. The exercise showcases this by aligning the chosen buffer systems with the required pH levels for the solution.

Acid-base buffer systems

Acid-base buffer systems are a mixture of a weak acid and its conjugate base or a weak base and its conjugate acid. These buffers play a pivotal role in resisting changes in pH when small quantities of an acid or base are added to the system. They work by neutralizing the added acid or base with the conjugate species, thus maintaining a relatively constant pH.

When preparing a buffer, it is not just about mixing any acid with any base. The chosen weak acid or base should have a pKa value near the desired pH, and it should be paired with a salt that provides the conjugate base or acid.

Selecting a Buffer Pair

In the exercise, for example, benzoic acid with a pKa of 4.19 was mixed either with sodium acetate or with HCl to create buffers at pH 4.0 and 5.0, respectively. This is in line with the buffer capacity principle where the greatest buffering effect occurs when the weak acid/base and its conjugate form are present in comparable amounts. This balance is struck effectively if we respect the pKa value and the intended pH target, ensuring that our system can proficiently withstand changes in pH upon addition of other substances.