Conductometric Titration Virtual Lab: HCl with NaOH

This conductometric titration virtual lab follows the conductivity titration of French, German and Australian senior chemistry courses. A conductivity cell sits in 20.0 mL of hydrochloric acid diluted with 180 mL of water. Students add sodium hydroxide 1 mL at a time, record the conductivity σ after each addition and plot σ against volume. The graph falls, then rises, and the equivalence point is where the two straight lines cross. No indicator is needed, and the molar ionic conductivities explain the shape. Every number below was read from the simulation.

Curriculum links

• French Terminale spécialité physique-chimie (constitution and transformations of matter): titration followed by conductimetry, and Kohlrausch's law σ = Σλᵢ[Xᵢ].
• German Abitur standards for chemistry (KMK): conductometric titration as a method of quantitative analysis.
• NSW HSC Chemistry, Module 6 (Acid/Base Reactions): quantitative analysis by titration, including conductivity graphs.

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Before the lab (5 min)

Ask: "You add sodium hydroxide solution to hydrochloric acid and measure the conductivity. Adding NaOH adds ions. What will the graph of conductivity against volume look like?" On a class link this is question 1; the simulation unlocks after students answer.

Method in the simulation

1. Open the Titration tab. Choose Titration HCl titrated with NaOH, set Concentration in beaker and Concentration in burette to 0.100 mol/L, and tick Add 180 mL distilled water and Fit two straight lines.
2. Press Record to log σ at 0 mL.
3. Press Auto-add 1 mL steps. It adds 1.0 mL and records σ each time. Press Pause at about 30 mL.
4. Read the equivalence volume where the two lines cross, then tick Plot dilution-corrected σ and read it again.
5. Press Fresh beaker, untick Add 180 mL distilled water and repeat without the correction.

V of NaOH (mL)	0	5	10	15	20	25	30
σ (mS/m)

Expected results

The meter fluctuates by about ±0.3 %, so values differ slightly between runs.

• Start: σ = 425–427 mS/m at 0 mL (model 425.9). It falls in a straight line to about 115 mS/m at 20 mL, then rises to about 218 mS/m at 30 mL.
• Two-line fit, with water: 19.73 and 19.76 mL for readings to 30 mL; 19.52 mL for readings to 50 mL. In 20,000 model runs to 30 mL, every fit fell between 19.7 and 19.8 mL.
• Dilution-corrected: 20.00 mL (19.99 in the browser), the true equivalence volume.
• Without the 180 mL of water: the uncorrected fit gave 16.49 mL, an 18 % error; the corrected fit gave 19.99 mL.

Questions for students

1. (Prediction, asked again after the lab) What shape is the graph of conductivity against volume of NaOH added?
2. Adding 180 mL of distilled water helps keep which variable almost constant?
3. What is σ at 0 mL?
4. Where do the two straight lines cross?
5. Explain the shape of the graph using the ions present.

Answers for teachers: (1) It falls to a minimum at the equivalence point, then rises. (2) The total volume, so dilution hardly changes the concentrations. (3) Accept 420–432 mS/m. (4) Accept 19.4–20.2 mL. (5) Before the equivalence point, each H⁺ (λ = 35.0) is replaced by Na⁺ (λ = 5.0), so σ falls. After it, the extra Na⁺ and OH⁻ (λ = 19.9) make σ rise. Cl⁻ stays throughout.

Common misconceptions

• "Adding ions always raises the conductivity." Here each OH⁻ added removes an H⁺, and the Na⁺ that stays conducts far less.
• "The conductivity is zero at the equivalence point." Sodium chloride solution still conducts: about 115 mS/m. Only H₂SO₄ titrated with Ba(OH)₂ falls almost to zero (0.3 mS/m at 20 mL), because both products leave the solution.
• "All ions conduct equally well." H⁺ and OH⁻ conduct several times better than Na⁺ or Cl⁻.
• "Dilution doesn't matter." Without the extra water the lines bend, and the uncorrected fit misses by over 3 mL.

Extension

• Other titrations: try CH₃COOH titrated with NaOH, NaCl titrated with AgNO₃ and H₂SO₄ titrated with Ba(OH)₂. Ask students to sketch each graph first.
• Kohlrausch's law: on the Conductivity cell tab, record sodium chloride at several concentrations to build a calibration line, then find the salt concentration of Water sample A and B.

FAQ

Can the class link open on the titration?

Yes. On the Share page, pin Screen to "Conductometric titration" in the link's starting values, with Titration, both concentrations and Add 180 mL of distilled water to the beaker. Otherwise step 1 tells students to open the tab.

Can I give each class an unknown concentration?

You can pin a different Concentration of the solution being titrated (20.0 mL) and tick Hide the concentration of the solution being titrated. On an ordinary link students can still see the pinned value in the starting values; lock the parameters (a Pro link option) to hide it.

Why is the fitted volume 19.8 mL and not 20.0 mL?

The volume grows from 200 to 230 mL during the titration, which bends the lines slightly. Ticking Plot dilution-corrected σ removes the effect.

Related simulations and guides

Acid–Base Titration and the Titration Curve Electrolysis of Aqueous Solutions with Inert Electrodes

For a titration with a burette and indicator, see the titration virtual lab and the titration of unknowns virtual lab.