Question

Which of the following, when added to water, form a buffered solution? (a) \(\mathrm{HF}\) and \(\mathrm{NaF}\) (b) \(\mathrm{HI}\) and \(\mathrm{KI}\) (c) \(\mathrm{HOCl}\) and \(\mathrm{NaOCl}\)

Short answer

Pairs (a) \(\mathrm{HF}\) and \(\mathrm{NaF}\), and (c) \(\mathrm{HOCl}\) and \(\mathrm{NaOCl}\) form a buffered solution when added to water.

Step-by-step solution

Analyze Pair (a)

Analyzing the pair \(\mathrm{HF}\) and \(\mathrm{NaF}\), we see that \(\mathrm{HF}\) is a weak acid and \(\mathrm{NaF}\) is the salt of its conjugate base. That being so, these would react with added H+ or OH- ions and keep the pH relatively stable, forming a buffered solution.

Analyze Pair (b)

Analyzing the pair \(\mathrm{HI}\) and \(\mathrm{KI}\), we see that \(\mathrm{HI}\) is a strong acid, not a weak one, and \(\mathrm{KI}\) is the salt of its conjugate base. As \(\mathrm{HI}\) is a strong acid, it will fully dissociate in solution, leaving no HI to 'soak up' any added base. Therefore, this combination would not form a buffered solution.

Analyze Pair (c)

Analyzing the pair \(\mathrm{HOCl}\) and \(\mathrm{NaOCl}\), we see that \(\mathrm{HOCl}\) is a weak acid and \(\mathrm{NaOCl}\) is the salt of its conjugate base. Therefore, just like in the first case, these would react with added H+ or OH- ions, forming a buffered solution.

Key concepts

Weak acid

When we talk about a weak acid, we are referring to an acid that does not completely ionize in solution. This distinct behavior is crucial in the formation of buffered solutions. Unlike strong acids, which dissociate fully, weak acids partially ionize, establishing an equilibrium between the acid and its ions.
For example, hydrogen fluoride (\(\mathrm{HF}\)) is considered a weak acid. In water, only a small fraction of \(\mathrm{HF}\) molecules ionize to form hydrogen ions (\(\mathrm{H}^+\)) and fluoride ions (\(\mathrm{F}^-\)). This partial dissociation means that the solution gets a measurable concentration of both the molecules and their ionized form.
Here are some key characteristics of weak acids:

• Their equilibrium constant, known as the \(K_a\) constant, is less than one, indicating incomplete ionization.
• This equilibrium allows the maintenance of pH even with the addition of small amounts of acids or bases.

Understanding weak acids is vital for recognizing buffered solutions since they are often paired with their conjugate base.

Conjugate base

The term conjugate base refers to what remains after a weak acid releases a hydrogen ion (proton). This concept is foundational in creating buffer systems alongside its paired weak acid.
Taking the example of hydrogen fluoride (\(\mathrm{HF}\)), when it donates a \(\mathrm{H}^+\) ion, it becomes a fluoride ion (\(\mathrm{F}^-\)). This fluoride ion is the conjugate base of \(\mathrm{HF}\). It can accept a hydrogen ion to re-form the weak acid, crucial for the buffer's function.
Characteristics of conjugate bases include:

• They are formed by the removal of a proton from a weak acid.
• They can combine with protons to form back the original weak acid, participating actively in neutralizing added acids or bases in a buffered solution.

This dual reaction capability allows the system to resist changes in pH, maintaining stability even after the addition of external substances that would otherwise alter the pH significantly.

pH stability

Buffered solutions are specially designed to maintain pH stability, meaning they can resist significant changes in pH upon the addition of acids or bases. This robustness is due to the presence of both a weak acid and its conjugate base.
The interplay between this weak acid and its conjugate base forms an equilibrium system. If additional hydrogen ions (acid) are introduced, the conjugate base will react with these ions to minimize pH changes. Similarly, if hydroxide ions (base) are added, the weak acid will react with these ions.
Benefits of such a system include:

• Preventing drastic shifts in pH that could be harmful in biological or chemical processes.
• Allowing processes that require a stable pH background to proceed efficiently.

Overall, buffer solutions are an essential tool in both laboratory and industrial settings, ensuring controlled and stable environmental conditions by maintaining consistent pH levels.