Dilute, connect, read, plot. The salt bridge carries ions.
A cell potential question in 9701 Paper 5 asks you to make solutions by dilution, set up two half-cells with a salt bridge and a high-resistance voltmeter, then use the Nernst equation as a straight line. Here a silver half-cell with changing [Ag⁺] is joined to a copper reference half-cell. Watch the set-up built step by step, then build it yourself and check it against the points the examiners look for.
Past-paper referenceOriginal experiment, wording and figures, written in the style of Cambridge AS & A Level Chemistry 9701 Paper 5. It is not copied from a past paper. The “what the examiners said” notes are our paraphrase of the Principal Examiner Reports for Teachers.
Practise next: 9701/54 May/June 2025, (cell potential against log of ion concentration, finding n) · official past papers
Practise next: 9701/51 October/November 2023, (making a solution by dilution in a volumetric flask) · official past papers
Practise next: 9701/51 October/November 2024, (a dilution series and explaining an anomalous point) · official past papers
Examiner reports used: 9701 March 2023, November 2023, June 2024, November 2024 and June 2025.
This set-up builder needs JavaScript.
What a full-mark diagram shows
The dilutions
Volumetric pipette (or burette) of stock into a 100.0 cm³ volumetric flask, make up to the mark, stopper and invert so the solution is uniform.
The cell
Each clean metal dips into a solution of its own ions; a salt bridge of filter paper soaked in KNO₃(aq) joins the beakers; a high-resistance voltmeter across the electrodes; temperature 25 °C.
The variables
Independent variable: the concentration of Ag⁺(aq), not lg[Ag⁺]. Dependent: Ecell. Control: the reference half-cell and the temperature.
The analysis
Plot Ecell against lg[Ag⁺]: gradient = 2.303RT/nF, intercept = E°cell. Round n to a whole number.
Habits the examiner reports keep asking for
The salt bridge moves ions, not electrons. Electrons go round the external circuit.
Use KNO₃ in the bridge, never KCl next to a silver half-cell: chloride precipitates silver chloride.
Name the concentration as the independent variable. lg[Ag⁺] is derived and was not credited.
Explain an anomaly with a real procedural error, such as too much water in that dilution, and repeat that reading to improve reliability.
n is a number of electrons, so give it as a whole number.