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4.1.2Physical Chemistry

Performing a Titration and Volumetric Analysis — AQA AS Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 96 of 102

📚 AQA AS Chemistry Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 6Terms and Apparatus

Burette
  • Burette Delivers the titrant, with 0.10 cm³ divisions and a reading uncertainty of ±0.05 cm³.
Volumetric pipette
  • Volumetric pipette Measures a fixed volume of solution, such as 25.0 cm³, into the conical flask.
Endpoint
  • Endpoint The point at which the indicator changes colour, showing the reaction is complete or very nearly so.
Equivalence point
  • Equivalence point The point at which the amount of titrant exactly matches the amount of substance in the flask.
Titre
  • Titre The volume delivered from the burette, equal to the final reading minus the initial reading.

Part 2 of 6Carrying Out the Titration

1
Use a volumetric pipette to put a known volume of one solution in a conical flask, and add a few drops of a suitable indicator.
2
Fill the burette with the other solution, near the 0.00 cm³ mark, and record the initial reading.
3
Run the titrant into the flask while swirling, with the flask standing on a white tile to make the colour change easier to see.
4
Add the titrant one drop at a time near the endpoint, and close the tap after the single drop that changes the colour.
5
Record the final reading, then repeat until two or more concordant titres are obtained.

Part 3 of 6Processing Results

Processing Results
Concordant titres
  • Concordant titres Titres that agree to within 0.10 cm³ are concordant and are averaged;
    • other titres are left out.
Titre uncertainty
  • Titre uncertainty Each burette reading is ±0.05 cm³, so the titre, from two readings, has an uncertainty of ±0.10 cm³.
Percentage uncertainty
  • Percentage uncertainty This is the absolute uncertainty divided by the measured value, multiplied by 100%.
Adding and subtracting
  • Adding and subtracting When values are added or subtracted, their absolute uncertainties are added together.

Part 4 of 6Good Practice

Good Practice
Rinsing
  • Rinsing Rinse the burette with the solution it will hold and the pipette with its own solution, to avoid dilution.
Funnel
  • Funnel Remove the funnel before the titration begins, because a drip from it would change the reading.
Air bubble
  • Air bubble Fill the burette tip so no air bubble remains, which would give a false volume.
Rough titration
  • Rough titration A first rough run shows where the endpoint is, so later runs can be added drop by drop near it.

Part 5 of 6Worked Example

Question

  • 25.0 cm³ of sodium hydroxide solution was titrated with 0.100 mol dm⁻³ hydrochloric acid.
  • The titres were 24.55, 24.60 and 24.65 cm³.
  • Calculate the concentration of the sodium hydroxide, and the percentage uncertainty in the mean titre.

  1. The titres agree within 0.10 cm³, so the mean titre is (24.55 + 24.60 + 24.65) ÷ 3 = 24.60 cm³.
  2. Moles HCl = 0.100 × 24.60 ÷ 1000 = 2.460 × 10⁻³ mol.
  3. The equation HCl + NaOH → NaCl + H₂O is 1:1, so moles NaOH = 2.460 × 10⁻³ mol.
  4. Concentration NaOH = 2.460 × 10⁻³ ÷ 0.0250 = 0.0984 mol dm⁻³.
  5. Percentage uncertainty in the titre = (0.10 ÷ 24.60) × 100 = 0.41%.

Answer The sodium hydroxide concentration is 0.0984 mol dm⁻³, and the titre has a percentage uncertainty of 0.41%.

Part 6 of 6Exam Checklist & Tips

4.1.2 – Key Points for the Exam

  • Titre The volume of solution run from the burette to reach the endpoint.
  • Concordant Titres agreeing within a stated small range, here 0.10 cm³.
  • Endpoint The point at which the indicator changes colour.
  • Absolute uncertainty The size of the possible error in a measurement, in the same units as that measurement.
  • Percentage uncertainty Absolute uncertainty divided by the measured value, expressed as a percentage.

Common Mistakes

  • Averaging all the titres, including a rough titre that is not concordant.
  • Quoting the titre uncertainty as ±0.05 cm³ when two readings are used, instead of ±0.10 cm³.
  • Confusing percentage uncertainty with percentage error, which compares a result with a literature value.
  • Leaving the funnel in the burette during the titration.

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