AP Chemistry Unit 8: Mechanisms and Calculations of Buffer Solutions

Buffer Solution

An aqueous system that resists changes in pH upon the addition of small amounts of strong acid or strong base.

pH Maintenance

A buffer maintains a constant pH relative to its specific chemical composition, not necessarily neutral (pH 7).

Buffer Composition

A buffer must contain an acidic component and a basic component that do not react with each other.

Conjugate Acid-Base Pair

Two components in a buffer that consist of a weak acid and its conjugate base or a weak base and its conjugate acid.

Direct Preparation of Buffer

Mixing a weak acid with a salt of its conjugate base, or a weak base with a salt of its conjugate acid.

Example of Direct Preparation

Acetic acid (HC₂H₃O₂) mixed with Sodium Acetate (NaC₂H₃O₂).

Partial Neutralization

Reacting a weak acid with a strong base, or vice versa, to create a buffer and generate the conjugate in situ.

Mechanism of Buffers

Buffers resist pH change by acting as a 'sink' for protons or hydroxide ions, following Le Châtelier’s Principle.

Addition of Strong Acid

When a strong acid is added to a buffer, the conjugate base consumes protons, preventing a drastic drop in pH.

Addition of Strong Base

When a strong base is added, the weak acid in the buffer consumes hydroxide, preventing a drastic increase in pH.

Henderson-Hasselbalch Equation

A formula that relates pH, pKₐ, and the ratio of concentrations of acid and conjugate base.

Henderson-Hasselbalch Formula

pH = pKₐ + log([A⁻]/[HA]).

pKₐ

The negative logarithm of the acid dissociation constant (Kₐ) of a weak acid.

When [A⁻] = [HA]

At this point, pH = pKₐ, occurring at the half-equivalence point of a titration.

When [A⁻] > [HA]

pH > pKₐ, meaning the buffer solution is more basic due to a higher concentration of conjugate base.

When [A⁻] < [HA]

pH < pKₐ, indicating that the solution is more acidic due to a higher concentration of weak acid.

Buffer Capacity

The amount of acid or base a buffer can neutralize before the pH changes significantly.

Factors Affecting Buffer Capacity

Absolute concentrations of acid and conjugate base, and the ratio of their concentrations.

High Capacity Buffer

A buffer with high concentrations (e.g., 1.0 M) that can absorb more protons or hydroxides.

Low Capacity Buffer

A buffer with low concentrations (e.g., 0.01 M) that is quickly exhausted when external acid/base is added.

Effective Buffer Range

A buffer is most effective within the range of pH = pKₐ ± 1.

'The Magic 7' Mistake

Misusing pH = 7 for buffers; buffers stabilize pH based on their pKₐ, not necessarily at neutral.

ICF Table Usage

A two-step calculation process for strong acid/base addition involving Initial, Change, and Final amounts.

Inverse Log Error

Flipping the ratio in the Henderson-Hasselbalch equation leads to incorrect calculations of pH.

Weak Acid + Strong Base

Can form a buffer if the weak acid is in excess; if moles are equal, it reaches equivalence, not a buffer.