Question

Which of the following is a buffer solution? (a) \(500 \mathrm{~mL}\) of \(0.1 \mathrm{~N} \mathrm{CH}_{3} \mathrm{COOH}+500 \mathrm{~mL}\) of \(0.1 \mathrm{~N} \mathrm{NaOH}\) (b) \(500 \mathrm{~mL}\) of \(0.1 \mathrm{~N} \mathrm{CH}_{3} \mathrm{COOH}+500 \mathrm{~mL}\) of \(0.1 \mathrm{~N} \mathrm{HCl}\) (c) \(500 \mathrm{~mL}\) of \(0.1 \mathrm{~N} \mathrm{CH}_{3} \mathrm{COOH}+500 \mathrm{~mL}\) of \(0.2 \mathrm{~N} \mathrm{NaOH}\) (d) \(500 \mathrm{~mL}\) of \(0.2 \mathrm{~N} \mathrm{CH}_{3} \mathrm{COOH}+500 \mathrm{~mL}\) of \(0.1 \mathrm{~N} \mathrm{NaOH}\)

Short answer

Option (d) is a buffer solution because it contains a mixture of a weak acid and its conjugate base in the form of a salt, capable of resisting changes in pH.

Step-by-step solution

Understanding Buffer Solutions

A buffer solution is one that contains a weak acid and its conjugate base in sufficient quantities to resist significant pH change when an acid or base is added. It is typically composed of a weak acid and a salt of its conjugate base.

Analyzing Option (a) for Buffer Properties

Solution (a) has equal concentrations (0.1 N) of a weak acid, acetic acid (CH}_{3}COOH), and a strong base (NaOH). When mixed, they will neutralize each other to form water and the salt CH}_{3}COONa, resulting in no excess weak acid or base, hence, it cannot act as a buffer.

Analyzing Option (b) for Buffer Properties

Solution (b) mixes acetic acid with a strong acid (HCl). There is no conjugate base present to form a buffer, hence this mixture will not function as a buffer.

Analyzing Option (c) for Buffer Properties

Solution (c) mixes acetic acid with a concentration that is half of the concentration of the strong base NaOH. The strong base will completely neutralize a portion of the weak acid, but since the base is in excess, no buffer system is formed.

Analyzing Option (d) for Buffer Properties

Solution (d) features a weak acid, acetic acid, at a higher concentration (0.2 N) than the strong base, NaOH (0.1 N). The base will neutralize half of the acetic acid, leaving behind acetic acid and the salt, sodium acetate (CH}_{3}COONa), which is the conjugate base. This solution will act as a buffer.

Key concepts

Acid-Base Neutralization

When discussing the fascinating world of chemistry, one concept that invariably pops up is 'acid-base neutralization'. This is a reaction where an acid and a base interact to form water and a salt. It's almost like they cancel each other out, with the acid’s sourness and the base’s bitterness giving way to a more neutral state. Think of it as a dance between two opposing characters who, upon meeting, step aside to let a new, less intense persona take the stage.

For an easier mental image, picture pouring vinegar, which contains acetic acid, into a solution of baking soda, or sodium bicarbonate. The fizz and foam you observe are the hallmarks of this neutralization. But, it’s not just for show in a kitchen science experiment; neutralization is key in various fields, from industrial processes to the very biological systems within us.

Conjugate Acid-Base Pairs

Moving onto 'conjugate acid-base pairs,' this concept anchors the renowned theory put forth by Bronsted and Lowry. A conjugate acid-base pair consists of two substances related by the loss or gain of a single hydrogen ion. While one member of the pair—known as the acid—donates the hydrogen ion, the other—called the base—accepts it.

Let's take water for a prime example. When it acts as a base and accepts a hydrogen ion, it becomes hydronium (\(H_3O^+\)), the conjugate acid. On the flip side, when it gives up a hydrogen ion and acts as an acid, it becomes hydroxide (\(OH^-\)), the conjugate base. This dynamic duo illustrates the fluid dance between acids and bases, highlighting the importance of balance and transformation in chemical reactions.

pH Stabilization

The final slice of our scientific pie is 'pH stabilization', which is a pivotal aspect in the study of substances' acidity or basicity. pH stands for 'potential hydrogen' and measures the concentration of hydrogen ions in a solution. It's a scale used to discern how acidic or basic a liquid is. Stabilizing pH means ensuring that the solution maintains its acidity or basicity levels, even when acids or bases are added to the mix. This stability is crucial in many industries like agriculture, medicine, and especially in the functioning of our bodies.

Enter buffer solutions—the peacekeepers of pH—they're specially crafted mixtures that resist dramatic swings in pH. By soaking up excess hydrogen or hydroxide ions, buffers keep the pH scale's delicate balance in check. Our blood, for example, owes its consistent pH to a natural buffer system, an elegant testament to nature’s foresight in sustaining life.