Question

Which one among the following mixtures gives a buffer solution in water?

• A. \(\text{CH}_3\text{COOH} + \text{CH}_3\text{COONa}\)
• B. \(\text{CH}_3\text{COOH} + \text{NaCl}\)
• C. \(\text{NaOH} + \text{NaCl}\)
• D. \(\text{NaOH} + \text{CH}_3\text{COONa}\)

Hint:

Buffer solutions are typically made from a weak acid and its conjugate base, or a weak base and its conjugate acid.

Answer 1

The Correct Option is A

A buffer solution is a solution that resists changes in pH when small amounts of an acid or a base are added. It is formed by a weak acid and its conjugate base or a weak base and its conjugate acid. The key feature of a buffer solution is its ability to neutralize small amounts of added acid (\(\text{H}^+\)) or base (\(\text{OH}^-\)) and maintain a relatively stable pH.
What Makes a Buffer?
A buffer solution typically consists of: - A weak acid (such as \(\text{CH}_3\text{COOH}\)) that partially dissociates in water to release \(\text{H}^+\) ions, and - The conjugate base of the weak acid (such as \(\text{CH}_3\text{COO}^-\), the acetate ion from \(\text{CH}_3\text{COONa}\)). In this case: - Acetic acid (\(\text{CH}_3\text{COOH}\)) is a weak acid, which dissociates slightly in water: \[ \text{CH}_3\text{COOH} \rightleftharpoons \text{CH}_3\text{COO}^- + \text{H}^+ \] - Sodium acetate (\(\text{CH}_3\text{COONa}\)) is a salt of the conjugate base, acetate (\(\text{CH}_3\text{COO}^-\)), and when dissolved in water, it dissociates completely into \(\text{Na}^+\) and \(\text{CH}_3\text{COO}^-\) ions: \[ \text{CH}_3\text{COONa} ⇒ \text{CH}_3\text{COO}^- + \text{Na}^+ \] The acetate ion (\(\text{CH}_3\text{COO}^-\)) acts as a conjugate base and can neutralize any excess \(\text{H}^+\) (acid) that is added to the solution: \[ \text{CH}_3\text{COO}^- + \text{H}^+ ⇒ \text{CH}_3\text{COOH} \] This helps in maintaining the pH of the solution. Similarly, if a base (\(\text{OH}^-\)) is added, the acetic acid can neutralize it: \[ \text{CH}_3\text{COOH} + \text{OH}^- ⇒ \text{CH}_3\text{COO}^- + \text{H}_2\text{O} \] Thus, the combination of acetic acid (\(\text{CH}_3\text{COOH}\)) and sodium acetate (\(\text{CH}_3\text{COONa}\)) forms a buffer solution.
Now, let us consider the other options: Option (B): \(\text{CH}_3\text{COOH} + \text{NaCl}\) - Sodium chloride (\(\text{NaCl}\)) is a neutral salt and does not provide any conjugate base (like acetate) to neutralize acids. Therefore, this mixture will not form a buffer solution. The pH will change when acid or base is added. Option (C): \(\text{NaOH} + \text{NaCl}\) - Sodium hydroxide (\(\text{NaOH}\)) is a strong base, and sodium chloride (\(\text{NaCl}\)) is a neutral salt. Mixing them will create a basic solution due to the dissociation of \(\text{NaOH}\), but it will not be a buffer solution because it does not contain both a weak acid and its conjugate base. The pH will rise if more base is added. Option (D): \(\text{NaOH} + \text{CH}_3\text{COONa}\) - Sodium hydroxide is a strong base, and sodium acetate is a conjugate base. Although this mixture will create a basic solution, it is not a buffer solution in the classical sense because it lacks a weak acid (like \(\text{CH}_3\text{COOH}\)) to neutralize any acids that might be added. It will act as a base but not a proper buffer.
Therefore, the correct answer is (A), where acetic acid and sodium acetate form a proper buffer solution.