Chapter 18: Aqueous Ionic Solutions

Which of the following combinations creates a buffer solution?

• Strong acid + Strong base

• Weak acid + Conjugate base

• Strong acid + Conjugate base

• Weak acid + Strong base

Answer explanation

A buffer solution is formed by a weak acid and its conjugate base, which can resist changes in pH. The other combinations do not provide this capability, making 'Weak acid + Conjugate base' the correct choice.

What happens when HCl is added to a buffer made of HC₂H₃O₂ and NaC₂H₃O₂?

• pH rises sharply

• HCl is neutralized by the acetate ion

• Buffer components are destroyed

• The solution becomes neutral

Answer explanation

When HCl is added to the buffer, the acetate ions (from NaC₂H₃O₂) react with HCl, neutralizing it. This maintains the pH of the solution, demonstrating the buffer's ability to resist changes in pH.

Select all that apply to buffer systems:

• • Contain significant amounts of weak acid and conjugate base

• • Resist pH changes

• • Always have pH = 7

• • Work through neutralization reactions

• • Require strong acid and strong base

Answer explanation

Buffer systems contain significant amounts of weak acid and its conjugate base, allowing them to resist pH changes. They can also work through neutralization reactions, but they do not always have a pH of 7.

5. Match each term with its correct description:

• Buffer

• -log of the acid dissociation constant

• Conjugate base

• Formed when acid donates a proton

• Weak acid

• Partially ionizes in wate

• pKa

• Resists changes in pH

7. Reorder the steps for calculating the pH of a buffer using the equilibrium approach:

• Substitute equilibrium expressions into the Ka equation

• Write the balanced equation and set up an ICE table

• Use pH = -log[H₃O⁺] to calculate pH

• Solve for [H₃O⁺] using Ka

Write the steps for calculating the pH of a buffer using the equilibirum approach.

Answer explanation

1. Identify the weak acid and its conjugate base in the buffer. 2. Write the equilibrium expression for the dissociation of the weak acid. 3. Use the Henderson-Hasselbalch equation: pH = pKa + log([A-]/[HA]). 4. Substitute concentrations to find pH.

Calculate the pH of a buffer that is 0.100 M in HC₂H₃O₂ and 0.100 M in NaC₂H₃O₂.
Ka = 1.8 × 10⁻⁵
Use the Henderson-Hasselbalch equation:
pH = pKa + log([base]/[acid])

Answer explanation

To find the pH, first calculate pKa: pKa = -log(Ka) = 4.74. Using the Henderson-Hasselbalch equation: pH = 4.74 + log(0.100/0.100) = 4.74. Thus, the pH of the buffer is 4.74.

What makes a buffer most effective?

• High concentrations of strong acids

• Equal concentrations of weak acid and conjugate base

• A very low pKa

• Dilute buffer solutions

Answer explanation

A buffer is most effective when it has equal concentrations of a weak acid and its conjugate base. This balance allows the buffer to resist changes in pH when small amounts of acid or base are added.

What happens if the ratio of base to acid in a buffer exceeds a factor of 10?

• The pH drops to 1

• The buffer is at peak performance

• The buffer becomes ineffective

• The solution neutralizes both acid and base equally

Answer explanation

When the ratio of base to acid in a buffer exceeds 10, the buffer's capacity to resist pH changes diminishes significantly, rendering it ineffective. Thus, the correct answer is that the buffer becomes ineffective.

Which solution would best maintain a buffer at pH = 4.25?

• HClO₂ (pKa = 1.95)

• HNO₂ (pKa = 3.34)

• HCHO₂ (pKa = 3.74)

• HClO (pKa = 7.54)

Answer explanation

To maintain a buffer at pH 4.25, we need a weak acid with a pKa close to this value. HCHO₂ (pKa = 3.74) is the closest option, making it the best choice for buffering at pH 4.25.