Core Practical 9: Determine Ka for a Weak Acid

Key Concepts

The weak acid partially dissociates in solution:

HA (aq) ⇌ H⁺ (aq) + A^– (aq)

The expression for the acid dissociation constant is:

Ka = [H⁺][A^–] / [HA]

At the half-neutralisation point, [HA] = [A–], so:

Ka = [H⁺]

To find [H⁺]: [H⁺] = 10^–pH

So: Ka = 10^–pH

Safety

• Wear a lab coat and eye protection.
• Tie back long hair.
• Sodium hydroxide solution is an irritant – handle with care.

Apparatus and Chemicals

Equipment

• pH meter or datalogger with probe
• 50 cm³ burette and stand
• 250 cm³ conical flask
• 25 cm³ pipette and filler
• Clamp and boss for pH probe

Chemicals

• 100 cm³ of 0.1 mol dm⁻³ ethanoic acid
• 100 cm³ of 0.1 mol dm⁻³ sodium hydroxide
• Phenolphthalein indicator

Procedure

1. Calibrate the pH meter or set up the datalogger.
2. Pipette 25.0 cm³ of ethanoic acid into the conical flask.
3. Fill a burette with 0.1 mol dm⁻³ sodium hydroxide solution.
4. Add 2–3 drops of phenolphthalein to the acid.
5. Titrate until pale pink persists (end point).
6. Add another 25.0 cm³ of ethanoic acid to the same flask.
7. Record the pH of the solution. At this point, half the acid has been neutralised.

Analysis

From the pH reading, calculate [H⁺]:

[H⁺] = 10^–pH

Since [HA] = [A–], Ka = [H⁺].

Example:
If pH = 4.77 at the half-equivalence point:
[H⁺] = 10^–4.77 = 1.7 × 10⁻⁵ mol dm⁻³
Ka = 1.7 × 10⁻⁵ mol dm⁻³
pKa = –log(1.7 × 10⁻⁵) = 4.77

Common Sources of Uncertainty

• Judging titration end point precisely.
• Reading burette incorrectly (always use bottom of meniscus on white tile).
• pH probe inaccuracies, ensure calibration.

Matt’s exam tip

A weak acid only partially ionises in solution. At half-neutralisation, [HA] = [A–] so Ka = [H+]. Record burette readings to 2 d.p. (e.g., 25.00 cm³).