Question

Describe how you would prepare a \(1-\mathrm{L} 0.20 \mathrm{M}\) \(\mathrm{CH}_{3} \mathrm{COONa} / 0.20 \mathrm{M} \mathrm{CH}_{3} \mathrm{COOH}\) buffer system by (a) mixing a solution of \(\mathrm{CH}_{3} \mathrm{COOH}\) with a solution of \(\mathrm{CH}_{3} \mathrm{COONa},\) (b) mixing a solution of \(\mathrm{CH}_{3} \mathrm{COOH}\) with a solution of \(\mathrm{NaOH}\), and (c) mixing a solution of \(\mathrm{CH}_{3} \mathrm{COONa}\) with a solution of \(\mathrm{HCl}\).

Short answer

Mix equal molar solutions of acids/bases and dilute to 1 L for each method.

Step-by-step solution

Understanding the Buffer System

The problem requires preparing a buffer system with equal concentrations of acetic acid \( \mathrm{CH}_3\mathrm{COOH} \) and sodium acetate \( \mathrm{CH}_3\mathrm{COONa} \) both at 0.20 M. A buffer system is designed to resist changes in pH upon the addition of small amounts of acids or bases.

Mixing Acetic Acid and Sodium Acetate Solutions

To prepare the buffer by mixing solutions of \( \mathrm{CH}_3\mathrm{COOH} \) and \( \mathrm{CH}_3\mathrm{COONa} \), you start with volumes of each solution that have the desired concentrations. Therefore, combine 0.20 L of 1.0 M \( \mathrm{CH}_3\mathrm{COOH} \) with 0.20 L of 1.0 M \( \mathrm{CH}_3\mathrm{COONa} \) and then dilute the mixture to a total volume of 1 L with distilled water.

Mixing Acetic Acid and Sodium Hydroxide Solutions

To prepare the buffer by mixing \( \mathrm{CH}_3\mathrm{COOH} \) and \( \mathrm{NaOH} \), start with an 0.40 L of 0.50 M \( \mathrm{CH}_3\mathrm{COOH} \). Then add 0.20 L of 0.50 M \( \mathrm{NaOH} \) since \( \mathrm{NaOH} \) fully neutralizes \( \mathrm{CH}_3\mathrm{COOH} \) in a 1:1 ratio, forming \( \mathrm{CH}_3\mathrm{COONa} \). Dilute the resulting mixture to 1 L with distilled water.

Mixing Sodium Acetate and Hydrochloric Acid Solutions

To prepare the buffer using \( \mathrm{CH}_3\mathrm{COONa} \) and \( \mathrm{HCl} \), start by using 0.40 L of 0.50 M \( \mathrm{CH}_3\mathrm{COONa} \). Add 0.20 L of 0.50 M \( \mathrm{HCl} \), which reacts with \( \mathrm{CH}_3\mathrm{COONa} \) to form \( \mathrm{CH}_3\mathrm{COOH} \). Finally, dilute this mixture to 1 L with distilled water.

Key concepts

Sodium Acetate

Sodium acetate, chemically denoted as \( \mathrm{CH}_3\mathrm{COONa} \), is the sodium salt of acetic acid. It plays a crucial role in the formation of buffer solutions. In a typical buffer system, such as the acetic acid/sodium acetate buffer described, sodium acetate acts as a conjugate base. It helps to neutralize any added acids, which helps maintain the pH level relatively stable.

To prepare the buffer, you would need a solution of sodium acetate, ideally at a concentration of 0.20 M as specified in the exercise. The accessibility of sodium acetate in laboratories makes it a convenient choice for preparing buffer solutions, especially since you can easily monitor and adjust its concentration to achieve the desired buffering capacity.

Sodium acetate is particularly effective when preparing buffer systems for biochemistry labs and industrial applications where the control of pH is vital. This is because it helps maintain the desired pH range with minimal shifts, making it very practical.

Acetic Acid

Acetic acid, known in chemistry as \( \mathrm{CH}_3\mathrm{COOH} \), is a weak acid with characteristic pungency and sourness. It is a key component in vinegar. In chemistry labs, acetic acid is often used for buffer preparation, especially in its aqueous form.

When creating the specified buffer system, you would need a solution of acetic acid at 0.20 M. This concentration allows it to pair effectively with sodium acetate to resist changes in pH. The equilibrium between acetic acid and sodium acetate in the buffer system, under the common-ion effect, stabilizes the mixture's pH by only slightly ionizing to form hydrogen ions and acetate ions.

It's important to understand that acetic acid leverages its equilibrium in water to create a balanced buffer solution when paired with its conjugate base—sodium acetate. This dynamic allows it to absorb excess hydrogen ions (if acid is added) and hydroxide ions (if a base is added), thus maintaining a pH close to its pKa, typically around 4.75.

pH Resistance

Buffer solutions are highly valued for their resistance to pH changes. This quality is particularly crucial when performing reactions that are sensitive to shifts in acidity or alkalinity. A buffer solution made from acetic acid and sodium acetate is a classic example of a system that provides this pH stability.

In this type of buffer, when an acid like \( \mathrm{HCl} \) is added, the acetate ion from sodium acetate can neutralize the added hydrogen ions, minimizing any pH decrease. Conversely, adding a base results in the acetic acid component neutralizing any extra hydroxide ions, preventing a significant pH increase.

This characteristic resilience to pH changes is underpinned by the buffer's capacity, strongly related to the concentrations of acetic acid and sodium acetate: higher concentrations offer greater resistance. It's a balancing act, wherein the relative amounts of the weak acid and its salt determine the buffer's effectiveness.

Neutralization Reaction

Neutralization reactions are central to understanding buffer solutions, particularly when considering how buffers resist changes in pH. These reactions involve an acid and a base reacting to form water and a salt, thereby 'neutralizing' the added reactants.

In the preparation of acetic acid/sodium acetate buffer, neutrality is achieved through complementary reactions, such as acetic acid with sodium hydroxide (NaOH) or sodium acetate with hydrochloric acid (HCl). During these reactions, NaOH can fully neutralize acetic acid, transforming it into sodium acetate, while HCl can convert sodium acetate into acetic acid.

This interplay allows the buffer solution to effectively balance added acids or bases, keeping the pH within a narrow range. Neutralization ensures that any acidic or basic disturbances are mitigated, illustrating how vital these reactions are in the creation and maintenance of effective buffer solutions.