Question

Which combinations of compounds can make a buffer? Assume aqueous solutions. a) \(\mathrm{H}_{3} \mathrm{PO}_{4}\) and \(\mathrm{Na}_{3} \mathrm{PO}_{4}\) b) \(\mathrm{NaHCO}_{3}\) and \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) c) \(\mathrm{NaNO}_{3}\) and \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\) d) \(\mathrm{HN}_{3}\) and \(\mathrm{NH}_{3}\)

Short answer

Combinations (a) and (b) can form buffers.

Step-by-step solution

Understanding Buffers

A buffer solution resists changes in pH upon the addition of small amounts of acid or base. It typically contains a weak acid and its conjugate base, or a weak base and its conjugate acid.

Analyzing Option a

Option (a) includes \(\mathrm{H}_3\mathrm{PO}_4\), which is a weak acid, and \(\mathrm{Na}_3\mathrm{PO}_4\), which is the sodium salt of the conjugate base of the acid. This pair can form a buffer solution.

Analyzing Option b

Option (b) includes \(\mathrm{NaHCO}_3\) and \(\mathrm{Na}_2\mathrm{CO}_3\). \(\mathrm{NaHCO}_3\) can act as a weak acid, and \(\mathrm{Na}_2\mathrm{CO}_3\) contains the conjugate base. Together, they can form a buffer solution.

Analyzing Option c

Option (c) includes \(\mathrm{NaNO}_3\) and \(\mathrm{Ca(NO}_3)_2\). Both are salts and do not have the components necessary for a buffer system, as \(\mathrm{NO}_3^-\) is the conjugate base of a strong acid (nitric acid) which does not act as a buffer.

Analyzing Option d

Option (d) includes \(\mathrm{HN}_3\) (hydrazoic acid), a weak acid, and \(\mathrm{NH}_3\) (ammonia), a weak base. However, together they do not form a buffer, as they do not share a conjugate pair relationship.

Conclusion

The combinations that can act as a buffer are (a) \(\mathrm{H}_3\mathrm{PO}_4\) and \(\mathrm{Na}_3\mathrm{PO}_4\) and (b) \(\mathrm{NaHCO}_3\) and \(\mathrm{Na}_2\mathrm{CO}_3\).

Key concepts

Weak Acid and Conjugate Base Pair

A buffer solution is a special type of solution that maintains a relatively constant pH even when small amounts of acids or bases are added. A key feature of these solutions is the presence of a weak acid and its conjugate base, or a weak base and its conjugate acid. This combination allows the buffer to neutralize added acids or bases, thus preventing significant shifts in pH.
For example, in the case of a weak acid like dihydrogen phosphate (\(\mathrm{H}_3\mathrm{PO}_4\)), adding its conjugate base sodium phosphate (\(\mathrm{Na}_3\mathrm{PO}_4\)) will create a situation where the weak acid can donate protons and the conjugate base can accept protons. This balance helps to stabilize the pH.

• A weak acid doesn't completely dissociate in water, meaning not all of its hydrogen ions are released.
• A conjugate base is the form a molecule takes after the weak acid has donated a proton.
• In our examples, only certain combinations like \(\mathrm{H}_3\mathrm{PO}_4\) and \(\mathrm{Na}_3\mathrm{PO}_4\) have the necessary weak acid-conjugate base relationship.

Understanding this interaction is crucial for designing effective buffer systems.

pH Resistance

One of the main roles of a buffer is to resist changes in pH levels. This is particularly important in both biological systems and chemical applications where maintaining a constant pH is essential.
When an acid (which increases hydrogen ion concentration) or a base (which decreases hydrogen ion concentration) is introduced to the solution, the buffer reacts with the added acid/base to minimize the change in pH.

• The weak acid present in a buffer solution will neutralize added bases by donating protons.
• Conversely, the conjugate base in a buffer will neutralize added acids by accepting protons.

For instance, a buffer composed of sodium bicarbonate (\(\mathrm{NaHCO}_3\)) and sodium carbonate (\(\mathrm{Na}_2\mathrm{CO}_3\)) will effectively resist pH changes when small amounts of acids or bases are added. The bicarbonate ion can act as a weak acid, and the carbonate ion as the conjugate base, both working together to stabilize the pH.

Aqueous Solutions

An aqueous solution is one where water is the solvent. This is a common medium for chemical reactions, including those involving buffer systems. The properties of water greatly influence how buffers operate, particularly due to its ability to dissolve a wide range of substances.
In aqueous solutions, water molecules help in the dissociation and interaction of the weak acid-conjugate base pairs, which is vital for buffer action.

• Water as a solvent can either aid or inhibit the functioning of acids and bases based on its hydrogen bonding capabilities.
• It provides a medium that facilitates the equilibrium necessary for the buffer’s resistance to pH change.
• Aqueous solutions are assumed in the context of our original exercise, as it's the typical setting for buffer action.

Understanding how buffer components interact in water is essential for mastering the concept of buffer solutions.