top of page

4.7–4.8 Titration and Titration Calculations

GCSE Chemistry · AQA · Chapter 4: Chemical Calculations

A free GCSE Chemistry lesson for AQA (8462) from OBA School: titration apparatus and method, concordant results, and calculating concentrations from titration results. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe how to carry out an acid–alkali titration accurately
  • Record burette readings, calculate titres and identify concordant results
  • Calculate a mean titre from concordant results
  • (Higher tier) use titration results and a balanced equation to calculate an unknown concentration in mol/dm³ and g/dm³

Key words

  • Titration: A method for finding the exact volumes of an acid and an alkali that react with each other.
  • End point: The point at which the indicator changes colour, showing the acid and alkali have just reacted completely.
  • Titre: The volume of solution added from the burette to reach the end point (final reading − initial reading).
  • Concordant results: Titres that are within 0.10 cm³ of each other.
  • Indicator: A substance that changes colour at the end point, e.g. phenolphthalein or methyl orange.

Titration apparatus

A titration measures the volume of acid needed to neutralise a known volume of alkali (or the other way round). The key pieces of equipment are chosen because they measure volumes very precisely.

  • Pipette (25.0 cm³) and pipette filler; Why it is used measures a fixed volume of alkali very accurately; the filler is used for safety
  • Burette; Why it is used adds acid gradually and measures the volume added, to the nearest 0.05 cm³
  • Conical flask; Why it is used can be swirled to mix without splashing
  • White tile; Why it is used makes the colour change at the end point easy to see
  • Indicator; Why it is used shows the end point. Phenolphthalein: pink in alkali → colourless in acid. Methyl orange: yellow in alkali → red in acid. Universal indicator is not used because its colour changes gradually.

Why exact amounts matter. When an acid and an alkali react they form a salt and water. The solution is only neutral if exactly the right amounts react. If there is acid left over the solution ends up acidic; if alkali is left over it ends up alkaline. The point at which the acid and alkali have just reacted completely is the end point, shown by the indicator changing colour.

Carrying out a titration

1. Use a pipette and pipette filler to put 25.0 cm³ of alkali into a clean conical flask: fill until the bottom of the meniscus is on the line, let it drain, and touch the tip on the inside of the flask. Do not blow out the small drop left in the tip – the pipette is calibrated for this. Add a few drops of indicator.

2. Stand the flask on a white tile under a burette filled with acid. Record the starting reading (read the bottom of the meniscus at eye level).

3. Add acid while swirling the flask. When the colour starts to change, add the acid drop by drop.

4. Stop as soon as the indicator changes colour permanently (the end point). Record the final reading.

5. The first run is a quick rough titration. Repeat accurately until you have at least two concordant titres (within 0.10 cm³).

Exam tip: Rinse the burette with a little of the acid and the pipette with a little of the alkali before using them, so the solutions are not diluted by leftover water.

Recording and processing results

  • Final reading (cm³); Rough 24.60; 1 24.20; 2 24.40; 3 24.40
  • Initial reading (cm³); Rough 0.00; 1 0.40; 2 0.10; 3 0.50
  • Titre (cm³); Rough 24.60; 1 23.80; 2 24.30; 3 23.90

Worked example: mean titre

Ignore the rough titration. Runs 1 and 3 (23.80 and 23.90 cm³) are within 0.10 cm³, so they are concordant. Run 2 (24.30 cm³) is not.
mean titre = (23.80 + 23.90) ÷ 2 = 23.85 cm³

Answer: 23.85 cm³

Titration calculations (Higher tier)

1. moles of the solution you know = concentration × volume (in dm³)

2. use the balanced equation’s ratio to find moles of the other substance

3. concentration of the other substance = moles ÷ volume (in dm³)

Worked example: sodium hydroxide and hydrochloric acid (1 : 1)

NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l). 25.0 cm³ of NaOH needed 20.00 cm³ of 0.100 mol/dm³ HCl.
moles HCl = 0.100 × 0.02000 = 0.00200 mol, so moles NaOH = 0.00200 mol
concentration NaOH = 0.00200 ÷ 0.0250 = 0.0800 mol/dm³
in g/dm³: 0.0800 × 40 = 3.20 g/dm³

Answer: 0.0800 mol/dm³ (3.20 g/dm³)

Worked example: sulfuric acid (1 : 2)

2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l). 25.0 cm³ of 0.120 mol/dm³ NaOH needed 15.00 cm³ of H₂SO₄.
moles NaOH = 0.120 × 0.0250 = 0.00300 mol
moles H₂SO₄ = 0.00300 ÷ 2 = 0.00150 mol
concentration H₂SO₄ = 0.00150 ÷ 0.01500 = 0.100 mol/dm³ (= 0.100 × 98 = 9.80 g/dm³)

Answer: 0.100 mol/dm³

Common mistakes

  • Including the rough titration in the mean. Use only concordant titres.
  • Forgetting to divide cm³ by 1000 before multiplying by concentration.
  • Ignoring the ratio in the equation: with H₂SO₄, 2 mol of NaOH react with 1 mol of acid.
  • Reading the top of the meniscus or reading from above or below. Read the bottom of the meniscus at eye level.
  • Using a measuring cylinder instead of a pipette to measure the alkali – it is much less accurate.

Revision questions

Try these, then check your answers below each one.

Question 1. Which piece of apparatus is used to add acid gradually and measure the volume added in a titration? (Choose from: Burette, Pipette, Measuring cylinder, Beaker.)

Answer: Burette

Working: A burette adds the acid gradually and measures the volume to the nearest 0.05 cm³. A pipette measures a fixed volume of alkali.

Question 2. What word describes titres that are within 0.10 cm³ of each other?

Answer: concordant

Working: Concordant results are within 0.10 cm³ of each other. Only these are used to work out the mean.

Question 3. A student records these titres (cm³): rough 23.00, then 22.40, 22.10 and 22.20.<br/>Calculate the mean titre from the concordant results.

Answer: 22.15 cm³

Working: Concordant: 22.10 and 22.20. Mean = (22.10 + 22.20) ÷ 2 = 22.15 cm³.

Related lessons

Continue in the full course

This lesson is section 4.7–4.8 (AGCCH0407) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.

 

 

 

 

 

 

 

bottom of page