AQA GCSE Chemistry Foundation (8462), Foundation tier · Chemical changes › Reactions of acids
Practise Titrations. 11 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
GCSE Chemistry only (not in Combined Science). How to carry out a titration to find the exact volumes of a strong acid and a strong alkali that react (required practical 2).
Key facts
Titre = final burette reading − initial burette reading
Concordant titres: within 0.10 cm3 of each other
cm3 ÷ 1000 = dm3
HCl + NaOH → NaCl + H2O
H2SO4 + 2NaOH → Na2SO4 + 2H2O
Notes
Method (required practical 2)
diagram
A titration finds the exact volumes of acid and alkali that react with each other.
1. Use a pipette and pipette filler to put exactly 25.0 cm3 of alkali (e.g. sodium hydroxide solution) into a conical flask.
2. Add a few drops of indicator (e.g. phenolphthalein or methyl orange) and stand the flask on a white tile.
3. Fill a burette with the acid and record the starting reading.
4. Add the acid while swirling the flask. Near the end point, add it drop by drop until the indicator just changes colour.
5. Record the final reading. Titre = final reading − initial reading.
6. Do a rough titration first, then repeat until you have concordant results (within 0.10 cm3 of each other).
Pipette the alkali into the flask; run the acid in from the burette until the indicator just changes colour.
Getting accurate results
Read the burette at eye level, at the bottom of the meniscus, to the nearest 0.05 cm3.
Use an indicator with a sharp colour change. Universal indicator is not used because its colour changes gradually.
The white tile makes the colour change easier to see.
Calculate the mean titre from the concordant results only, not the rough titre.
How to answer each type of question
6-mark: describe how to carry out a titration
4 to 6 marksGrade 5
Measure a fixed volume of alkali with a pipette into a conical flask and add indicator.
Fill the burette with acid and record the initial reading.
Add acid while swirling, dropwise near the end point, until the colour just changes.
Record the final reading, repeat to get concordant results and calculate the mean.
Example. Describe how you would use a titration to find the volume of dilute hydrochloric acid that reacts exactly with 25.0 cm3 of potassium hydroxide solution. [6 marks]
Show the model answerHide the model answer
Marked in levels. A top-level answer is a complete method in a logical order, such as: Use a pipette (and filler) to measure 25.0 cm3 of potassium hydroxide solution into a conical flask (1) Add a few drops of indicator, e.g. phenolphthalein, and stand the flask on a white tile (1) Fill a burette with hydrochloric acid and record the reading (1) Add the acid while swirling, drop by drop near the end point (1) Stop when the indicator changes colour (pink to colourless) and record the final reading (1) Repeat until results are concordant (within 0.10 cm3) and calculate the mean (1)
Calculate a mean titre
2 marksGrade 5
Pick the concordant titres (within 0.10 cm3 of each other).
Leave out the rough titre and any result that is not concordant.
Add the concordant titres and divide by how many there are; give 2 decimal places.
Example. A student recorded these titres.
Titration
Rough
1
2
3
4
Titre in cm3
24.60
23.95
24.30
24.00
23.90
Calculate the mean titre using only the concordant results.Show the model answerHide the model answer
Concordant titres are 23.95, 24.00 and 23.90 cm3 (1) Mean = (23.95 + 24.00 + 23.90) ÷ 3 = 71.85 ÷ 3 = 23.95 cm3 (1)
Don’t lose marks
Including the rough titre in the mean.
Reading the burette from the top of the meniscus, or not at eye level.
Writing burette readings to 1 decimal place, e.g. 23.5 instead of 23.50.
More tips
Memory tricks
Burette readings are written to 2 decimal places, ending in 0 or 5, e.g. 23.45 cm3.
Exam technique
Show every step of a calculation with units: you can still get marks for correct working if the final answer is wrong.
Give answers to a sensible number of significant figures, usually the same as the data (often 3).
Use the technical words: pipette, burette, conical flask, end point, titre, concordant.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 4
Name the equipment used in a titrationBurette, pipette with pipette filler, conical flask, indicator and white tile.
Grade 5
Describe how to carry out a titrationAdd acid from a burette to a measured volume of alkali with indicator until the colour just changes.
Grade 5
Choose concordant results and find the meanUse titres within 0.10 cm3 of each other and leave out the rough titre.
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Name the piece of apparatus used to measure exactly 25.0 cm3 of sodium hydroxide solution.
(volumetric) pipette
Name the piece of apparatus used in a titration to add the acid to the alkali a drop at a time near the end point.
burette
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
A student used a titration to find the volume of hydrochloric acid needed to neutralise 25.0 cm3 of sodium hydroxide solution.
(a) Name the piece of apparatus used to measure exactly 25.0 cm3 of sodium hydroxide solution.[1]
(b) Name the piece of apparatus used to add the hydrochloric acid.[1]
(c) The student added phenolphthalein indicator to the sodium hydroxide solution. Give the colour change at the end point.[1]
(d) Suggest why universal indicator is not used in titrations.[1]
Show the answer and mark scheme
(a)Answer: (volumetric) pipette
(volumetric) pipette
(b)Answer: burette
burette
(c)Answer: pink to colourless
pink to colourless
(d)
it changes colour gradually (through many colours), so there is no sharp end point
Question 2Medium5 marks
A student carried out a titration to find the volume of sulfuric acid that reacts exactly with 25.0 cm3 of potassium hydroxide solution. The acid was added from a burette to the alkali in a conical flask.
(a) Why did the student rinse the burette with some of the sulfuric acid before filling it?[1]
(b) Why did the student place the conical flask on a white tile?[1]
(c) Why did the student swirl the flask during the titration?[1]
(d) Why did the student add the acid one drop at a time near the end point?[1]
(e) Describe how the student should read the burette.[1]
Show the answer and mark scheme
(a)
to remove any water / other substances that would dilute (or react with) the acid
(b)
so the colour change of the indicator is easier to see
(c)
to mix the acid and alkali (so they react completely)
(d)
so the end point is not overshot / to find the exact volume needed
(e)
with the eye level with the liquid, reading the bottom of the meniscus