Titration Calculations
Section: 3. Stoichiometry | Syllabus: Cambridge IGCSE Chemistry (0620)
Acid-Base Titration
A titration finds the exact volume of one solution needed to react completely with a known volume of another - most often used to find the concentration of an unknown acid or alkali.
| Equipment | Purpose |
|---|---|
| Pipette | measures a fixed, precise volume into the conical flask |
| Burette | delivers a variable, precisely-read volume of the other solution |
| Conical flask | holds the solution being titrated, swirled throughout |
| Indicator | changes colour sharply at the end point |
- End point: the point at which the indicator just changes colour, showing the reaction is complete
- Titre: the volume of solution added from the burette to reach the end point
- Concordant results: titres that agree within 0.10 cm³ of each other - these are averaged (a rough, less precise first titre is not included)
A white tile under the flask makes the indicator's colour change at the end point easier to see
Calculation Method
Volume → Moles → use mole ratio from equation → Moles → Concentration
- Write the balanced equation
- Calculate moles of the solution with known concentration (moles = concentration × volume)
- Use the mole ratio from the equation to find moles of the unknown solution
- Calculate the concentration of the unknown (concentration = moles ÷ volume)
Worked Example: Finding an Unknown Concentration
- Question: 25.0 cm³ of sodium hydroxide solution is exactly neutralised by 20.0 cm³ of 0.1 mol/dm³ hydrochloric acid. NaOH + HCl → NaCl + H₂O. Calculate the concentration of the sodium hydroxide. [3]
- Answer: moles HCl = 0.1 × (20.0÷1000) = 0.002 mol [1]; ratio NaOH:HCl is 1:1, so moles NaOH = 0.002 mol [1]; concentration NaOH = 0.002 ÷ (25.0÷1000) = 0.08 mol/dm³ [1]
Worked Example: Unequal Mole Ratio
- Question: 25.0 cm³ of sulfuric acid is exactly neutralised by 30.0 cm³ of 0.2 mol/dm³ sodium hydroxide. H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O. Calculate the concentration of the acid. [3]
- Answer: moles NaOH = 0.2 × (30.0÷1000) = 0.006 mol [1]; ratio H₂SO₄:NaOH is 1:2, so moles H₂SO₄ = 0.006 ÷ 2 = 0.003 mol [1]; concentration H₂SO₄ = 0.003 ÷ (25.0÷1000) = 0.12 mol/dm³ [1]
Averaging Concordant Titres
| Trial | Titre (cm³) | Use in average? |
|---|---|---|
| Rough | 24.50 | No |
| 1 | 24.15 | Yes |
| 2 | 24.20 | Yes |
| 3 | 24.18 | Yes |
Trials 1-3 are concordant (all within 0.10 cm³ of each other), so average them: (24.15 + 24.20 + 24.18) ÷ 3 = 24.18 cm³. The rough titre is excluded because it is only an estimate used to judge how quickly to add acid in later, careful trials.
Indicators and Improving Accuracy
| Indicator | Acid colour | Alkali colour |
|---|---|---|
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red | Yellow |
- Use a white tile under the flask to make the colour change easier to see
- Add the solution from the burette dropwise, swirling constantly, as the end point approaches
- Read the burette scale at eye level to avoid parallax error
- Repeat until concordant results are obtained, and average only those
Common Mistakes
- ❌ Averaging the rough titre along with the concordant results
- ✓ only average titres that are concordant (within 0.10 cm³ of each other) - the rough titre is a guide, not data
- ❌ Forgetting to convert cm³ to dm³ before calculating moles
- ✓ divide every volume by 1000 before using moles = concentration × volume
Interactive revision notes, videos and practice questions load below.