Question

Aqueous solutions of \(\mathrm{HNO}_{3}, \mathrm{KOH}, \mathrm{CH}_{3} \mathrm{COOH}\), and \(\mathrm{CH}_{3}\) COONa of identical concentrations are provided. The pair(s) of solutions which form a buffer upon mixing is(are) A) \(\mathrm{HNO}_{3}\) and \(\mathrm{CH}_{3} \mathrm{COOH}\) B) \(\mathrm{KOH}\) and \(\mathrm{CH}_{3} \mathrm{COONa}\) C) \(\mathrm{HNO}_{3}\) and \(\mathrm{CH}_{3} \mathrm{COONa}\) D) \(\mathrm{CH}_{3} \mathrm{COOH}\) and \(\mathrm{CH}_{3} \mathrm{COONa}\)

Short answer

The solutions that form a buffer upon mixing are D) CH3COOH and CH3COONa, since they consist of a weak acid and its corresponding salt.

Step-by-step solution

Identify the Components of a Buffer Solution

Understand that a buffer solution consists of a weak acid or a weak base and its corresponding salt (which contains its conjugate base or conjugate acid). The mixture's ability to resist changes in pH when small amounts of acid or base are added depends on the presence of these two components.

Evaluate Given Solution Pairs

Analyze each pair of compounds to determine if they can make a buffer solution by checking if they consist of a weak acid and its corresponding salt, or a weak base and its corresponding salt. Strong acids, like HNO3, and strong bases, like KOH, do not make buffer solutions when paired with salts.

Select the Correct Pair(s)

Eliminate the pairs that contain strong acids or strong bases. Check remaining pairs for the presence of a weak acid/base and its salt. In this case, CH3COOH (acetic acid) is a weak acid, and CH3COONa (sodium acetate) is its salt. Therefore, they create a buffer solution when mixed.

Conclude with the Correct Answer

Option D (CH3COOH and CH3COONa) forms a buffer solution because it is a mixture of a weak acid and its corresponding salt.

Key concepts

Acid-Base Equilibrium

Understanding acid-base equilibrium is crucial when studying chemistry, particularly when it comes to buffer solutions. Acid-base equilibrium refers to the state where the rates of the forward and reverse reactions between acids and bases are equal, resulting in a stable pH level.

At the heart of this concept is the dissociation of acids and bases in water. When an acid dissociates, it donates a proton (H⁺), while a base accepts a proton. This process is reversible, and at equilibrium, the concentrations of all reactants and products remain constant.

The equilibrium constant for this reaction, known as the dissociation constant (K_a for acids and K_b for bases), provides insight into the strength of an acid or base. Weak acids, like acetic acid (CH₃COOH), only partially dissociate in water, which is fundamental for creating a buffer solution along with its conjugate base in the form of a salt, like sodium acetate (CH₃COONa).

pH Stability in Solutions

pH stability in solutions is often maintained through the use of buffers. The pH scale measures the acidity or basicity of a solution, with lower values being more acidic, higher values being more basic, and a pH of 7 being neutral. The ability of a buffer solution to maintain a stable pH, even after the addition of an acid or a base, is essential in many biological and chemical applications.

Buffer solutions achieve this stability by using a weak acid and its conjugate base or a weak base and its conjugate acid. The weak acid can neutralize added bases, while the conjugate base can neutralize added acids. This dual action allows the solution to resist changes in pH, maintaining an environment where chemical reactions can proceed predictably.

For instance, the buffering action of acetic acid and sodium acetate ensures that if an acid is added to the solution, the acetate ions (CH₃COO⁻) can react with H⁺ ions, while if a base is added, acetic acid can donate H⁺ ions, thus keeping the pH relatively unchanged.

Weak Acid and Conjugate Base

The pairing of a weak acid with its conjugate base forms the backbone of a buffer solution. A weak acid is an acid that does not fully dissociate in water, meaning it does not release all of its hydrogen ions into the solution. Instead, it reaches an equilibrium where both the undissociated acid and the conjugate base coexist.

The conjugate base is the species that remains after the weak acid has donated a proton. In the context of buffer solutions, the conjugate base is typically provided in the form of a salt, which when dissolved in water, increases the concentration of the conjugate base ions. For example, when sodium acetate (CH₃COONa) dissolves, it yields acetate ions (CH₃COO⁻), the conjugate base of acetic acid.

This combination is effective because the weak acid can neutralize added bases while the conjugate base can neutralize added acids, as previously mentioned. Their capability to maintain a steady pH when faced with small amounts of additional acidic or basic substances is what makes them particularly valuable in physiological processes and industrial applications where pH control is vital.