Problem 26 - Chemistry The Molecular Science

Question

If added to \(1 \mathrm{~L}\) of \(0.20-\mathrm{M}\) acetic acid, \(\mathrm{CH}_{3} \mathrm{COOH},\) which of these would form a buffer? (a) \(0.10 \mathrm{~mol} \mathrm{NaCH}_{3} \mathrm{COO}\) (b) \(0.10 \mathrm{~mol} \mathrm{NaOH}\) (c) \(0.10 \mathrm{~mol} \mathrm{HCl}\) (d) \(0.30 \mathrm{~mol} \mathrm{NaOH}\) Explain your answers.

Step-by-Step Solution

Answer

Options (a) and (b) form a buffer.

Step 1: Understanding Buffer Solutions

A buffer solution consists of a weak acid and its conjugate base or a weak base and its conjugate acid. It is resistant to changes in pH when small amounts of an acid or a base are added.

Step 2: Identifying Acetic Acid and Its Conjugate Base

Acetic acid (\( \text{CH}_3\text{COOH} \)) is a weak acid. Its conjugate base is acetate, \( \text{CH}_3\text{COO}^- \). To form a buffer with acetic acid, you need the presence of acetate ions.

Step 3: Analyzing Option (a)

Option (a) adds \( 0.10 \text{ mol NaCH}_3\text{COO} \), which provides acetate ions \((\text{CH}_3\text{COO}^-)\). Since both \( \text{CH}_3\text{COOH} \) and \( \text{CH}_3\text{COO}^- \) are present, a buffer is formed.

Step 4: Analyzing Option (b)

Option (b) adds \( 0.10 \text{ mol NaOH} \), which reacts with \( \text{CH}_3\text{COOH} \): \( \text{CH}_3\text{COOH} + \text{OH}^- \rightarrow \text{CH}_3\text{COO}^- + \text{H}_2\text{O} \). This produces acetate ions, forming a buffer.

Step 5: Analyzing Option (c)

Option (c) adds \( 0.10 \text{ mol HCl} \), which fully dissociates to donate \( \text{H}^+ \) ions and reacts with \( \text{CH}_3\text{COO}^- \) if present. However, it does not provide \( \text{CH}_3\text{COO}^- \), so no buffer is formed.

Step 6: Analyzing Option (d)

Option (d) adds \( 0.30 \text{ mol NaOH} \). This amount significantly exceeds \( 0.20 \text{ mol} \) of \( \text{CH}_3\text{COOH} \) and converts all of it into \( \text{CH}_3\text{COO}^- \), destroying the balance needed for a buffer.

Key Concepts

Acetic Acid

Acetic acid is a prominent example of a weak acid. In chemistry, a weak acid is one that doesn't completely dissociate in solution, which means not all of its molecules separate into ions. This property is essential for its participation in buffer solutions. Acetic acid is structurally represented as \( \text{CH}_3\text{COOH} \), where its functional group, \(-\text{COOH} \), is what defines its acidic property. This part of the molecule can donate a hydrogen ion (H^+), thereby categorizing it as an acid.
For a buffer solution to be effective, the presence of this weak acid is crucial because it can moderate pH changes through its interaction with its conjugate base. In our scenario, scenarios like adding sodium acetate (\text{NaCH}_3\text{COO}) introduce acetate ions to interact with acetic acid. Their balance plays a vital role in buffer capacity.

Conjugate Base

The conjugate base in a buffer solution involving acetic acid is acetate, \( \text{CH}_3\text{COO}^- \). A conjugate base forms when a weak acid, like acetic acid, loses a hydrogen ion (H^+). This transformative ability is the crux of how buffers maintain pH stability.
When we add an alkali substance like sodium hydroxide (\text{NaOH}), the hydroxide ions (\text{OH}^-) react with acetic acid to generate water and acetate ions, effectively forming the conjugate base. The presence of both the acid and its conjugate base allows the buffer to enhance pH resistance. Understanding how conjugate bases work is crucial when predicting whether a solution will resist changes in acidity or basicity, especially when slight amounts of strong acids or bases are added.

Weak Acid

A weak acid, by its very nature, partially dissociates in solution, which was exemplified with acetic acid in our buffer problem. Unlike strong acids that completely ionize in water, weak acids establish equilibrium between their ionized and non-ionized forms. This means there's always some amount of undissociated acetic acid along with its ions in solution, allowing it to donate hydrogen ions ( H^+ ) when necessary.
Weak acids are essential in the formation of buffer systems because their partial dissociation ensures that both the acid form and the conjugate base remain in balance. This balance is critical for maintaining pH levels within narrow ranges. For example, when a small amount of a strong base is introduced, a buffer neutralizes the base; likewise, when a strong acid is added, it is partially neutralized by the weak acid.

pH Resistance

pH resistance is a hallmark of buffer solutions. It is the ability of a solution to maintain a near-constant pH despite additions of acids or bases. This characteristic is particularly valuable in biological systems where enzymes and biochemical processes require stable pH levels to function properly.
The mechanism behind pH resistance lies in the buffer's composition: a weak acid paired with its conjugate base, or vice versa. When an acid is added to the solution, the conjugate base neutralizes it; similarly, the presence of the weak acid allows for the neutralization of added bases. In our original exercise, adding sodium acetate results in the formation of acetate ions, which buffer against bases, while the remaining acetic acid buffers against acids. This dynamic balance grants the buffer solution its resilience to pH changes.