Pearson International A Level Chemistry Core Practical 11: Determine the acid dissociation constant of a weak acid

This core practical is part of Pearson's practical-learning programme. The method below is one defensible implementation, not a confidential instruction sheet. Centres may adapt equipment while preserving the scientific purpose, controls, measurements and safety requirements.

Aim and scientific principle

Determine acid dissociation behaviour from pH and concentration data while accounting for logarithms and equilibrium assumptions.

Begin by translating the title into a testable question. Identify what is deliberately changed, what is measured directly, what is calculated and what must remain controlled. Predict a relationship only after naming the mechanism. A prediction without a reason is weak; a reason without measurable variables cannot guide a practical design.

Apparatus and setup

Use weak acid solution, calibrated pH meter, volumetric glassware, standard alkali if titrating, stirrer and temperature measurement. Select every measuring instrument by range and resolution, not by name alone. Draw a labelled scientific setup that shows the working geometry and exactly where readings are taken. Include a control or blank when it distinguishes the target effect from drift, contamination or background response.

Before collecting results, check zero readings, calibration, leaks, alignment or sterility as relevant. Record the instrument resolution beside the first raw-data table. If a digital device displays more digits than its underlying accuracy supports, do not treat every digit as equally certain.

Method

1. Calibrate the pH meter with suitable buffers and rinse the electrode between solutions.
2. Measure weak-acid concentration and pH at controlled temperature, or collect a careful titration curve.
3. Convert pH to hydrogen-ion concentration and apply the appropriate equilibrium expression.
4. Repeat and compare Ka values, checking whether approximations are justified.

Plan the analysis before starting. This reveals whether an additional measurement is needed for a derived quantity and prevents a completed experiment from lacking a mass, length, concentration, background or calibration value. A method should be detailed enough for another trained student to reproduce, while avoiding decorative steps that do not affect validity.

Variables and controls

• Independent variable: acid concentration or titrant volume.
• Dependent variable: pH and derived hydrogen-ion concentration.