General Chemistry — Buffers and the Henderson-Hasselbalch Equation

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Theory — Buffers and the Henderson-Hasselbalch Equation

A buffer is a solution of a weak acid and its conjugate base (or a weak base and its conjugate acid) in comparable amounts. The pair neutralises added acid or base, so the pH barely moves.

1. How a buffer works

The weak acid HA neutralises added base, and its conjugate base A^- neutralises added acid. Because both are present in reserve, small additions are absorbed with only a tiny pH change.

2. The Henderson-Hasselbalch equation

Taking the logarithm of the weak-acid equilibrium gives a direct formula for buffer pH:

Henderson-Hasselbalch pH = pK_a + log([A^-] / [HA])
pK_a = −log K_a
When [A^-] = [HA], the ratio is 1, log 1 = 0, and pH = pK_a.

This is why the half-equivalence point of a titration, where half the acid has been neutralised, gives pH = pK_a.

3. Preparing a buffer

To make a buffer at a chosen pH, pick a weak acid whose pK_a is close to that pH, then set the ratio of conjugate base to acid from the equation. A ratio between about 1:10 and 10:1 keeps the buffer effective, so a buffer works best within roughly one pH unit of its pK_a.

4. Buffer capacity

Buffer capacity is how much acid or base a buffer can absorb before the pH shifts sharply. It is greatest when the acid and base concentrations are high and their ratio is near 1. Push the ratio too far, or exhaust one component, and the buffer fails.

Apparatus

Buffer work uses tools to measure pH and to deliver acid, base, and buffer components precisely. In the simulation these are modelled, but the readings match what each instrument would give.

Buffer beaker

A beaker holding a weak acid and its conjugate base in equilibrium.

pH meter

Reads the pH of the solution directly as acid or base is added.

Burette

Delivers a measured volume of strong acid or base into the solution.

Acid and base bottles

Stock strong acid and strong base used to test how the buffer responds.

Volumetric pipette

Delivers exact volumes of the acid and conjugate base to set the buffer ratio.

Titration plot

A pH curve showing the flat buffer region around the pK_a.

Instructions

Work through both tabs. Calculate first by hand, then press the button to compare.

Part A — Buffer pH from Henderson-Hasselbalch

Choose a weak acid (its pK_a is shown) and set the concentrations of the acid HA and its conjugate base A^-.
Compute pH = pK_a + log([A^-]/[HA]) by hand.
Enter your pH and press Check; it compares within 0.05 pH units.

Part B — Buffer in action

Add a small amount of strong acid or base to a buffered and an unbuffered solution.
Predict which solution holds its pH, then press Check.

Simulation

Weak acid

[HA] (M)

[A⁻] (M)

Your pH

Add strong acid/base (mmol)

Which holds its pH?

Team Questions

1. When [A^-] equals [HA], the buffer pH equals:

7.00 always 0 pK_a 14 − pK_a

2. A buffer works best when the target pH is:

Far from the pK_a Within about one unit of the pK_a Exactly 7 Above 12 only

3. A buffer resists pH change because it contains:

Both a weak acid and its conjugate base in reserve Only a strong acid Only water A catalyst

Example Report

Worked example: the pH of an acetate buffer

An acetic-acid buffer has pK_a = 4.74, with [HA] = 0.10 M and [A^-] = 0.20 M.

pH = pK_a + log([A^-]/[HA]) = 4.74 + log(0.20/0.10) = 4.74 + log(2) = 4.74 + 0.30 = 5.04.

The buffer sits at pH 5.04, just above its pK_a because there is more conjugate base than acid. Choosing the ratio to hit a target pH is the calculate-then-compare core of the lab.

Buffers and the Henderson-Hasselbalch Equation