Conductivity – Kohlrausch's law and conductometric titration
ChemistryAcids, Bases & TitrationAges 16–17
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Sign in to playA virtual practical with a conductivity meter. On the Conductivity cell screen, change the electrolyte and its concentration, read the conductivity σ, test Kohlrausch's law σ = Σλᵢ[Xᵢ], compare the molar conductivity of strong and weak electrolytes, and build a NaCl calibration line to find the salt concentration of two water samples. On the Titration screen, add NaOH to HCl or ethanoic acid, NH₃ to HCl, AgNO₃ to NaCl or Ba(OH)₂ to H₂SO₄, record σ against volume in a table and graph, and find the equivalence point where two straight lines meet.
Lesson: Electrolytes and conductivity: strong and weak electrolytes, molar conductivity, Kohlrausch's law, conductometric titration
What it shows
A solution conducts electricity because its ions move in an electric field. The conductivity σ is the sum of the contributions of all the ions: σ = Σλᵢ[Xᵢ], where λᵢ is the molar ionic conductivity (Kohlrausch's law of independent migration). H⁺ and OH⁻ conduct far better than other ions. Strong electrolytes are fully ionised, so σ rises almost in proportion to concentration; weak electrolytes such as ethanoic acid are only partly ionised. In a conductometric titration, ions of one kind are replaced by others or removed, so σ changes along two straight lines that meet at the equivalence point, with no indicator needed.
How to use
On the Conductivity cell tab, choose a Solution, set the Concentration and press Record. Record sodium chloride at several concentrations to build a calibration line, then record Water sample A and B to read their concentrations. Switch Graph to Λm against √c (Kohlrausch) to compare strong and weak electrolytes. On the Titration tab, choose the Titration, add titrant with + 1.0 mL or Auto-add 1 mL steps and record σ after each addition. Tick Fit two straight lines to find the equivalence volume.
Parameters you can change
- Screen Conductivity cell, Kohlrausch's law, Conductometric titration
- Solution in the conductivity cell Hydrochloric acid HCl, Sodium hydroxide NaOH, Potassium chloride KCl, Sodium chloride NaCl (standard), Silver nitrate AgNO₃, Sodium ethanoate CH₃COONa, Ethanoic acid CH₃COOH (weak), Ammonia NH₃ (weak), Water sample A, Water sample B
- Electrolyte concentration 0.1–100 mmol/L
- Titration HCl titrated with NaOH, CH₃COOH titrated with NaOH, HCl titrated with NH₃, NaCl titrated with AgNO₃, H₂SO₄ titrated with Ba(OH)₂
- Concentration of the solution being titrated (20.0 mL) 0.01–0.2 mol/L
- Concentration of the titrant in the burette 0.05–0.2 mol/L
- Add 180 mL of distilled water to the beaker
- Hide the concentration of the solution being titrated
Questions to explore
- Why does σ fall at first when sodium hydroxide is added to hydrochloric acid, even though ions are being added?
- Why does the conductivity almost reach zero when barium hydroxide neutralises sulfuric acid?
- Why is a calibration line needed to find the salt concentration of a water sample?
