2.3.5Unit 2. Energetics, Group Chemistry, Halogenoalkanes and Alcohols
Acid-Base Titrations with Indicators — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 25 of 53 Unit quiz
Curriculum Equation Key pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: pH + pOH = 14
Part 1 of 5Indicators
An indicator is a weak acid or base whose conjugate forms have different colours.
Litmus
- Litmus is red in acid and blue in alkali, but its change is gradual so it is not suited to titrations.
Methyl orange
- Methyl orange is red in acid and yellow in alkali, with an orange end point.
Phenolphthalein
- Phenolphthalein is colourless in acid and pink in alkali, so it gives a sharp, easily seen change.
Choice of burette
- With phenolphthalein, put the alkali in the burette so the end point is the first permanent pink, easier to see than the loss of colour.
End point
- The end point is the volume at which the indicator changes colour, and it should coincide with the equivalence point.
| Indicator | Colour in acid | Colour in alkali |
|---|---|---|
| Litmus | Red | Blue |
| Methyl orange | Red | Yellow |
| Phenolphthalein | Colourless | Pink |
Part 2 of 5Carrying Out a Titration
1
Rinse the burette with the solution it will contain, fill it, and run some through the tip to remove air bubbles.
2
Use a volumetric pipette to transfer a fixed volume of the solution into a conical flask and add a few drops of indicator.
3
Add titrant while swirling, slowly near the end point, and record the first reading as the rough titre.
4
Repeat with fresh portions, adding drop by drop near the end point, until titres agree within 0.10 cm³.
5
Calculate the mean of the concordant titres only, and use it in the mole calculation.
Part 3 of 5Calculations and Uncertainty
Calculations and Uncertainty
Moles
- Amount in mol equals concentration in mol dm⁻³ multiplied by volume in dm³, so divide cm³ by 1000.
Ratio
- Use the balanced equation to link the moles of the known solution to the moles of the unknown.
Percentage uncertainty
- Divide the total uncertainty by the measured value and multiply by 100.
Two readings
- A burette reading is taken twice, so the absolute uncertainty on a titre is double that of one reading.
Part 4 of 5Worked Example
Question
- 25.0 cm³ of 0.100 mol dm⁻³ NaOH is neutralised by 20.00 cm³ of dilute sulfuric acid.
- Find the concentration of the acid, and the percentage uncertainty of the titre if each burette reading has an uncertainty of ±0.05 cm³.
- Equation2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O.
- Moles of NaOH = 0.100 × 0.0250 = 2.50 × 10⁻³ mol.
- Moles of H₂SO₄ = 2.50 × 10⁻³ ÷ 2 = 1.25 × 10⁻³ mol.
- Concentration = 1.25 × 10⁻³ ÷ 0.02000 = 0.0625 mol dm⁻³.
- Titre uncertainty = 2 × 0.05 = ±0.10 cm³, so % uncertainty = 0.10 ÷ 20.00 × 100 = 0.50%.
Answer The acid concentration is 0.0625 mol dm⁻³, and the titre has a percentage uncertainty of 0.50%.
Part 5 of 5Exam Checklist & Tips
2.3.5 – Key Points for the Exam
- Titration A technique for finding the concentration of a solution by reacting it with a solution of known concentration.
- Titre The volume of solution added from the burette to reach the end point.
- Concordant Results that agree closely, usually within 0.10 cm³.
- Equivalence point The point at which the reactants have been added in exactly the ratio shown by the equation.
Common Mistakes
- Using the rough titre when calculating the mean titre.
- Forgetting to convert cm³ to dm³ before multiplying by concentration.
- Ignoring the reacting ratio, for example treating H₂SO₄ as reacting 1 : 1 with NaOH.
- Counting only one burette reading's uncertainty when the titre needs two readings.
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