Question

Which one of the following pair show buffer's solution? (a) \(\mathrm{NaCl}+\mathrm{NaOH}\) (b) \(\mathrm{CH}_{3} \mathrm{COONa}+\mathrm{CH}_{3} \mathrm{COOH}\) (c) \(\mathrm{CH}_{3} \mathrm{COOH}+\mathrm{CH}_{3} \mathrm{COONH}_{4}\) (d) \(\mathrm{H}_{2} \mathrm{SO}_{4}+\mathrm{CuSO}_{4}\)

Short answer

Option (b) CH₃COONa + CH₃COOH is a buffer solution.

Step-by-step solution

Understanding Buffer Solution

A buffer solution is one that resists changes in pH when small amounts of an acid or a base are added. It is usually made up of a weak acid and its conjugate base, or a weak base and its conjugate acid.

Examine Option (a)

Option (a) presents the combination of NaCl and NaOH. NaCl is a salt and NaOH is a strong base. This combination does not form a buffer solution since it does not include a weak acid and its conjugate base or a weak base and its conjugate acid.

Examine Option (b)

Option (b) involves the pair CH₃COONa and CH₃COOH. CH₃COONa is a salt of the weak acid CH₃COOH and its conjugate base CH₃COO⁻. This combination fulfills the criteria for a buffer solution as it consists of a weak acid and its conjugate base.

Examine Option (c)

In option (c), CH₃COOH (a weak acid) and CH₃COONH₄ are given. CH₃COONH₄ can be treated as a salt, but it does not provide the conjugate base necessary for forming a buffer with CH₃COOH. Thus, this set does not fulfill the buffer criteria.

Examine Option (d)

Option (d) combines H₂SO₄ and CuSO₄. H₂SO₄ is a strong acid and CuSO₄ is just a salt. This pair does not form a buffer solution because it doesn’t consist of a weak acid/base pair with its conjugate.

Conclusion

After evaluating all options, only option (b) CH₃COONa and CH₃COOH fulfills the requirements for a buffer solution. It comprises a weak acid (CH₃COOH) and its conjugate base (CH₃COO⁻).

Key concepts

Weak Acid and Conjugate Base

A buffer solution is crucial in chemical reactions due to its ability to maintain a stable pH. It typically consists of a weak acid and its conjugate base or a weak base and its conjugate acid.
- A weak acid does not fully dissociate in water, allowing some molecules to remain in their original form. This incomplete dissociation is what makes an acid 'weak'. Examples include acetic acid (\(\mathrm{CH}_3\mathrm{COOH}\)). - The conjugate base is the species formed when the weak acid donates a proton. In acetic acid's case, the conjugate base is acetate (\(\mathrm{CH}_3\mathrm{COO}^-\)).
The combination of a weak acid and its conjugate base allows the buffer to neutralize small quantities of added acids or bases, maintaining the solution’s pH within a narrow range.

pH Resilience

Buffer solutions are designed to resist changes in pH when small amounts of additional acid or base are introduced. This property, known as pH resilience, is vital for processes that are sensitive to pH changes.
- When an acid is added to a buffer, the conjugate base in the buffer will react with the extra hydronium ions (\(\mathrm{H}^+\)) from the acid. This neutralizes the effect of the added acid.- Conversely, if a base is added, the weak acid component of the buffer will donate protons to neutralize the hydroxide ions (\(\mathrm{OH}^-\)) introduced by the base.
This intricate balance allows biological systems and laboratory processes to maintain consistent conditions important for proper chemical functioning.

Chemical Equilibrium

In a buffer solution, the concept of chemical equilibrium plays a significant role. It is the state where the concentrations of reactants and products remain constant over time. This dynamic equilibrium is essential for the buffer's ability to resist pH changes.
- The weak acid and its conjugate base in the buffer are in a state of equilibrium, described by the equilibrium equation: \[\mathrm{HA} \rightleftharpoons \mathrm{H}^+ + \mathrm{A}^-\]- Here, \( \mathrm{HA} \) stands for the weak acid, while \( \mathrm{A}^- \) represents the conjugate base.
When the equilibrium is disturbed by the addition of more \( \mathrm{H}^+ \) or \( \mathrm{OH}^- \) ions, Le Chatelier's Principle states that the system will adjust to counteract the change and restore equilibrium, thus stabilizing the pH.

Acid-Base Chemistry

Understanding the principles of acid-base chemistry is essential to grasp how buffer solutions work. This area of chemistry deals with the transfer of protons (hydrogen ions).
- Acids are proton donors. They release hydrogen ions into the solution. - Bases are proton acceptors. They accept hydrogen ions, usually forming water in the process.
In buffer solutions, the weak acid donates a proton to substances that are more basic, while the conjugate base picks up protons from substances that are more acidic. This back-and-forth exchange of protons helps maintain a relatively constant pH level, which is especially important in biological systems where even small pH changes can be detrimental.