Question

Which of the following solutions can act as a buffer: (a) \(\mathrm{KCl} / \mathrm{HCl}\) (b) \(\mathrm{KHSO}_{4} / \mathrm{H}_{2} \mathrm{SO}_{4}\) (c) \(\mathrm{Na}_{2} \mathrm{HPO}_{4} / \mathrm{NaH}_{2} \mathrm{PO}_{4}\) (d) KNO, /HNO?? ?

Short answer

Na2HPO4 / NaH2PO4 is the buffer solution.

Step-by-step solution

Define a Buffer Solution

A buffer solution is one that can resist changes in pH upon the addition of small amounts of an acid or a base. It is generally composed of a weak acid and its conjugate base, or a weak base and its conjugate acid.

Analyze Option (a) - KCl / HCl

The solution KCl / HCl consists of potassium chloride (KCl), which is a neutral salt, and hydrochloric acid (HCl), which is a strong acid. Since there is no weak acid or conjugate base pair, this is not a buffer solution.

Analyze Option (b) - KHSO4 / H2SO4

The solution KHSO4 / H2SO4 consists of potassium hydrogen sulfate (KHSO4), which doesn't form a buffer with sulfuric acid (H2SO4) since both components are strong acids and lack a weak acidic component or its conjugate base, hence this is not a buffer solution.

Analyze Option (c) - Na2HPO4 / NaH2PO4

The solution Na2HPO4 / NaH2PO4 consists of disodium hydrogen phosphate (Na2HPO4), which acts as a weak base, and sodium dihydrogen phosphate (NaH2PO4), which acts as a weak acid. This pair can function together to resist changes in pH, acting as a buffer solution.

Analyze Option (d) - KNO, / HNO

There seems to be a typo in the chemical notations. However, assuming the intention was KNO3 (potassium nitrate) and HNO3 (nitric acid), both are strong substances, thus cannot form a buffer solution. KNO3 is a neutral salt while HNO3 is a strong acid.

Conclusion

Based on the analysis, Na2HPO4 / NaH2PO4 is the only pair that consists of a weak acid and its conjugate base, making it capable of acting as a buffer solution.

Key concepts

Weak Acid

Understanding the concept of a weak acid is crucial in the realm of chemistry, especially when discussing buffer solutions. A weak acid is an acid that doesn't completely dissociate into its ions in a solution. This partial ionization is what gives weak acids their unique properties.
For example, acetic acid \((\text{CH}_3\text{COOH})\) is a classic weak acid. In water, it partially ionizes to produce hydrogen ions \( \text{H}^+ \) and acetate ions \( \text{CH}_3\text{COO}^- \).

• Weak acids are characterized by their relatively low ionization constant, known as \( K_a \).
• They contribute to the stability of a buffer solution.
• Weak acids play a key role in maintaining the \( pH \) in buffer systems.

In a buffer solution, the weak acid helps neutralize added bases, thus resisting significant \( pH \) changes.

Conjugate Base

The conjugate base is formed when a weak acid donates a hydrogen ion \( (\text{H}^+) \). This relationship between a weak acid and its conjugate base is essential for the operation of buffer solutions.
For instance, when acetic acid \( ( \text{CH}_3\text{COOH} ) \) loses a hydrogen ion, acetate \( ( \text{CH}_3\text{COO}^- ) \) becomes the conjugate base.

• Conjugate bases can accept hydrogen ions, thus neutralizing added acids.
• They work alongside weak acids to maintain buffer solution \( pH \).
• They have a direct impact on a solution's ability to resist \( pH \) change.

By working in tandem with a weak acid, a conjugate base helps sustain a constant \( pH \) in a buffer system.

pH Resistance

The ability to resist \( pH \) changes is a defining feature of buffer solutions. This quality is known as \( pH \) resistance and is critically important in many biochemical processes.
Buffer solutions resist changes in \( pH \) by neutralizing added acids (via their conjugate bases) and neutralizing added bases (via their weak acids).

• They stabilize pH during chemical reactions where \( pH \) fluctuations could cause problems.
• In biological systems, buffers help maintain the pH within narrow limits, essential for proper biological function.
• Commonly used in laboratory settings to maintain consistent reaction conditions.

Ultimately, the \( pH \) resistance arises because of the presence of both a weak acid and its conjugate base.

Phosphate Buffer System

The phosphate buffer system is one of the major buffer systems used in biological and chemical applications. It plays a significant role in maintaining the \( pH \) inside cells and in extracellular fluids.
This system typically consists of a mixture like sodium dihydrogen phosphate \( \text{NaH}_2\text{PO}_4 \) (weak acid) and disodium hydrogen phosphate \( \text{Na}_2\text{HPO}_4 \) (conjugate base). Together, they form a solution capable of resisting \( pH \) changes.

• The phosphate buffer system works within the \( pH \) range of 6.8 to 7.4.
• It is crucial in cellular functions where enzyme activity is pH dependent.
• This buffer is particularly effective at stabilizing the pH in the biological range.

The phosphate buffer system is a prominent example used in the original exercise, illustrating how a weak acid and its conjugate base work together to maintain a stable \( pH \) environment.