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4.1.3Organic Chemistry

Determining Concentrations — Edexcel AS Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 35 of 42

📚 Edexcel AS Chemistry Hub / Topical Revision
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Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
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Part 1 of 6Performing a Titration

A titration finds the volume of one solution that exactly reacts with a fixed volume of another.

1
Rinse the burette with the solution it will contain, then fill it and record the initial reading to the nearest 0.05 cm³.
2
Use a volumetric pipette to transfer 25.0 cm³ of the other solution into a conical flask standing on a white tile.
3
Add a few drops of a suitable indicator to the flask, then run the burette solution in while swirling the flask.
4
Slow to dropwise addition near the end point and stop when a single drop produces a permanent colour change.
5
Record the final reading, work out the titre, and repeat until two or more titres agree within 0.10 cm³.

Part 2 of 6Processing the Results

Titre
  • Titre Volume delivered from the burette equals final reading minus initial reading.
Rough titre
  • Rough titre A first quick run that shows roughly where the end point lies and is not used in the mean.
Concordant results
  • Concordant results Titres that agree within 0.10 cm³ of each other, which are averaged to get the mean titre.
Burette uncertainty
  • Burette uncertainty Each reading has an uncertainty of ±0.05 cm³, so a titre has ±0.10 cm³ because two readings are used.
Percentage uncertainty
  • Percentage uncertainty Uncertainty ÷ measured value × 100, which shows how good a measurement is relative to its size.

Part 3 of 6From Titre to Concentration

From Titre to Concentration
Moles of known solution
  • Moles of known solution n = concentration × volume in dm³, using the standard solution's volume.
Mole ratio
  • Mole ratio Use the balanced equation to convert moles of one reactant to moles of the other.
Unknown concentration
  • Unknown concentration c = moles ÷ volume in dm³ of the solution being analysed.
Significant figures
  • Significant figures Give the answer to the same number of significant figures as the least precise data.

Part 4 of 6Indicators and Accuracy

Methyl orange
  • Methyl orange Changes from yellow to red as acid is added, suitable for strong acid with weak base.
Phenolphthalein
  • Phenolphthalein Changes from pink to colourless as acid is added, suitable for weak acid with strong base.
Rinsing glassware
  • Rinsing glassware Rinse the burette with its own solution and the conical flask with water only, because extra water does not alter moles.
End point
  • End point Reading the burette at eye level and waiting for a permanent colour avoids over-shooting.

Part 5 of 6Worked Example

Question

  • 25.0 cm³ of 0.100 mol dm⁻³ Na₂CO₃ is titrated with hydrochloric acid.
  • The mean titre is 24.80 cm³.
  • Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂.
  • Calculate the concentration of the acid.

  1. Moles of Na₂CO₃ = 0.100 × 0.0250 = 0.00250 mol.
  2. The ratio is 1 : 2, so moles of HCl = 2 × 0.00250 = 0.00500 mol.
  3. Volume of acid = 24.80 ÷ 1000 = 0.02480 dm³.
  4. Concentration = 0.00500 ÷ 0.02480 = 0.2016 mol dm⁻³, which is 0.202 mol dm⁻³ to 3 s.f.
  5. Percentage uncertainty in the titre = 0.10 ÷ 24.80 × 100 = 0.40%.

Answer 0.202 mol dm⁻³

Part 6 of 6Exam Checklist & Tips

4.1.3 – Key Points for the Exam

  • Titre The volume of solution added from the burette to reach the end point.
  • End point The point at which the indicator changes colour, showing the reaction is complete.
  • Concordant Results that agree within 0.10 cm³ of each other.
  • Indicator A substance that changes colour near the end point of a titration.

Common Mistakes

  • Averaging all the titres including the rough one, which lowers accuracy.
  • Using a 1 : 1 ratio when the balanced equation shows 1 : 2.
  • Quoting the titre uncertainty as ±0.05 cm³ instead of ±0.10 cm³.
  • Rinsing the pipette or burette with water rather than the solution it will deliver.

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