Ch.17 - Additional Aspects of Aqueous Equilibria

Problem 3

Chapter 17, Problem 3

A buffer contains a weak acid, HA, and its conjugate base. The weak acid has a pKa of 4.5, and the buffer has a pH of 4.3. Without doing a calculation, state which of these possibilities are correct at pH 4.3. (a) 3HA4 = 3A-4, (b) 3HA4 7 3A-4, or (c) 3HA4 6 3A-4. [Section 17.2]

Verified step by step guidance

Understand the relationship between pH, pKa, and the concentrations of the weak acid and its conjugate base using the Henderson-Hasselbalch equation: \( \text{pH} = \text{pKa} + \log \left( \frac{[A^-]}{[HA]} \right) \).

Since the pH (4.3) is less than the pKa (4.5), the logarithmic term \( \log \left( \frac{[A^-]}{[HA]} \right) \) must be negative.

A negative logarithm indicates that the ratio \( \frac{[A^-]}{[HA]} \) is less than 1, meaning \([A^-] < [HA]\).

This implies that the concentration of the weak acid, HA, is greater than the concentration of its conjugate base, A^-.

Therefore, the correct possibility is (b) \([HA] > [A^-]\).

Verified Solution

Video duration:

0m:0s

This video solution was recommended by our tutors as helpful for the problem above.

Was this helpful?

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Buffer Solutions

A buffer solution is a system that resists changes in pH upon the addition of small amounts of acid or base. It typically consists of a weak acid and its conjugate base, which work together to neutralize added acids or bases. Understanding how buffers maintain pH is crucial for analyzing the behavior of the weak acid HA and its conjugate base in the given scenario.

Henderson-Hasselbalch Equation

The Henderson-Hasselbalch equation relates the pH of a buffer solution to the pKa of the weak acid and the ratio of the concentrations of the conjugate base to the weak acid. It is expressed as pH = pKa + log([A-]/[HA]). This equation helps predict the relative amounts of HA and A- in the buffer at a given pH, which is essential for determining the correct relationship among the concentrations in the question.

pKa and pH Relationship

The pKa of a weak acid is the pH at which the concentrations of the acid (HA) and its conjugate base (A-) are equal. When the pH is lower than the pKa, the solution contains more HA than A-. Conversely, if the pH is higher than the pKa, the solution has more A-. In this case, since the pH (4.3) is below the pKa (4.5), we can infer that the concentration of HA is greater than that of A-, which is key to answering the question.

Recommended video:

Guided course

02:09

pH and pOH Calculations

Related Practice

Open Question

Tooth enamel is composed of hydroxyapatite, whose simplest formula is Ca5(PO4)3OH, and whose corresponding Ksp = 6.8 × 10^(-27). As discussed in the Chemistry and Life box on page 746, fluoride in fluorinated water or in toothpaste reacts with hydroxyapatite to form fluoroapatite, Ca5(PO4)3F, whose Ksp = 1.0 × 10^(-60). (b) Calculate the molar solubility of each of these compounds.

Textbook Question

The beaker on the right contains 0.1 M acetic acid solution with methyl orange as an indicator. The beaker on the left contains a mixture of 0.1 M acetic acid and 0.1 M sodium acetate with methyl orange. (a) Using Figures 16.8 and 16.9, which solution has a higher pH?

746

views

Textbook Question

The beaker on the right contains 0.1 M acetic acid solution with methyl orange as an indicator. The beaker on the left contains a mixture of 0.1 M acetic acid and 0.1 M sodium acetate with methyl orange. (b) Which solution is better able to maintain its pH when small amounts of NaOH are added? Explain. [Sections 17.1 and 17.2]

642

views

Textbook Question

The following diagram represents a buffer composed of equal concentrations of a weak acid, HA, and its conjugate base, A-. The heights of the columns are proportional to the concentrations of the components of the buffer. (a) Which of the three drawings, (1), (2), or (3), represents the buffer after the addition of a strong acid? [Section 17.2]

377

views

Textbook Question

The following diagram represents a buffer composed of equal concentrations of a weak acid, HA, and its conjugate base, A-. The heights of the columns are proportional to the concentrations of the components of the buffer. (c) Which of the three represents a situation that cannot arise from the addition of either an acid or a base? [Section 17.2]

694

views

Textbook Question

The following figure represents solutions at various stages of the titration of a weak acid, HA, with NaOH. (The Na+ ions and water molecules have been omitted for clarity.) To which of the following regions of the titration curve does each drawing correspond: (b) after addition of NaOH but before the equivalence point? [Section 17.3]

1274

views