Launch practical
About 60 minutes
- Set up the bench with the 11 items of apparatus.
- Take the readings yourself, into the results table you draw first.
- Process your results.
- Answer exam questions on your own results, which you mark against the mark scheme.
Runs in your browser: a 3D bench with 2D close-ups of each instrument for exact readings. Nothing to install and no account.
What you measure, and how
One run takes 12 readings. You take every one yourself, the way you would in class.
- On the 3D bench (Fig. 1) you set out the apparatus and read each instrument where it stands.
- In the 2D close-up (Fig. 2) the same instrument is drawn large, from the same run, for an exact reading. It is also how the practical runs on a device that cannot show 3D.
- In your table you write what you read, to the instrument’s resolution, and the notebook checks it against the instrument.
Your eye does the reading. Each volume starts with your eye off level, so the scale lines up with the wrong mark. You move your eye, on the bench or in the close-up, until it is level. Nothing is right by default.
What it shows
2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l)
Acid neutralises alkali: H⁺ ions react with OH⁻ ions to make water. The indicator changes colour at the end point, when the alkali has just been neutralised. From the volume of acid needed (the titre) and the known concentration of the alkali, you can work out the concentration of the acid: moles = concentration × volume in dm³.
The graph to expect. No graph is drawn: the accurate titres are compared, and the concordant ones (within 0.10 cm³) are averaged.
Key equation for the exam: concentration = amount ÷ volume (concentration in mol/dm³ = amount in mol ÷ volume in dm³).
Recall: learn it · Higher tier. Divide volumes in cm³ by 1000 first.
Apparatus
| Item | What it is for | Resolution |
|---|---|---|
| Safety goggles | Protect your eyes from the alkali and the acid. | — |
| 50 cm³ burette | Adds the acid and measures how much was added. | 0.05 cm³ |
| Clamp stand with a burette clamp | Holds the burette vertical over the flask. | — |
| Small funnel | For filling the burette. | — |
| 25.0 cm³ pipette and pipette filler | Measures exactly 25.0 cm³ of alkali. | ±0.06 cm³ |
| 250 cm³ conical flask | Holds the alkali; can be swirled without spilling. | — |
| White tile | Makes the colour change easy to see. | — |
| Sodium hydroxide solution, 0.100 mol/dm³ | The alkali of known concentration. | — |
| Dilute sulfuric acid (concentration unknown) | The acid you are finding the concentration of. | — |
| Indicators in dropping bottles | Methyl orange, phenolphthalein and universal indicator. | — |
| Labelled beakers and a wash bottle of distilled water | For the acid, the alkali, waste and rinsing. | — |
Method
In our own words, as the Lab runs it. Your school's method sheet may differ in detail.
Which version are you doing?
Exam boards set this titration with different solutions. Your course setting picks one; you can change it before you start.
Why: The burette, pipette and technique are the same; which solution is unknown, the calculation and the exam questions follow your board.
Rinse the burette with the acid
Pour in about 5 cm³ of the acid, tilt and turn the burette so it wets the inside, then run it out through the tap into a waste beaker.
Why: Water left inside from washing would dilute the acid you fill it with.
Rinse the pipette with the alkali
With the filler, draw a little sodium hydroxide solution into the pipette, tilt it to wet the inside, and let it run to waste.
Why: Water left inside would dilute the alkali you measure out.
Rinse the conical flask
Choose what to rinse the conical flask with before each titration.
Do: Distilled water (it can stay wet).
Why: Only the alkali you measure with the pipette should react with the acid.
Fill the burette at a low levelSafety
Clamp the burette vertically, about halfway up the stand, with the tap closed. Lower the stand onto a stool or the floor so the top of the burette is below your eye level, then fill it through the small funnel to just above the 0.00 line.
Why: Pouring acid above head height can run it down the burette onto your face and into your eyes.
Fill the jet below the tap
Open the tap fully for a moment over the waste beaker so acid fills the jet below it and pushes out any air bubble. Then let the level fall to just below the 0.00 line.
Why: An air bubble in the jet can come out during a titration: the reading falls, but no acid went into the flask.
Take the funnel out
Lift the small funnel out of the top of the burette before you read it.
Why: Acid left in the funnel can drip into the burette after you have read it.
Choose how to measure the alkali
You need exactly the same volume of sodium hydroxide solution, 25.0 cm³, in every titration.
Do: 25.0 cm³ volumetric pipette and filler.
Why: The acid’s concentration is worked out from the moles of alkali, so the alkali’s volume has to be accurate and the same each time.
Choose the indicator
Add 5 drops of indicator to the alkali in the flask each time.
Do: Methyl orange.
Why: The indicator shows the end point: the moment the alkali has just been neutralised.
Stand the flask on a white tile
Put a white tile on the base of the stand, under the burette, and stand the conical flask on it.
Why: Against white, the first permanent colour change is much easier to see.
Do a rough titration first
In the first titration, run the acid in quickly while swirling, just to find roughly where the end point is. Do not use it in the mean.
Why: Then in the accurate titrations you can add acid quickly to about 1 cm³ before the end point, and go drop by drop from there.
Take the readings
A rough titration, then accurate titrations until at least two titres are concordant (within 0.10 cm³). Two burette readings each.
Repeats: Do a rough titration, then repeat accurately until at least two titres are concordant (within 0.10 cm³ of each other). The mean of the concordant titres reduces the effect of random errors, such as judging the end point; it does not remove systematic ones, such as not rinsing the burette with acid.
Make safe and clear upSafety
Run the acid left in the burette into a beaker, rinse the burette with water and clamp it upside down with the tap open. Rinse the pipette with water and put it back in its rack. Pour the neutralised solutions from the flasks down the sink with plenty of water. Wipe up any drips with a damp cloth, then take off your goggles and wash your hands.
While you take readings
Read the burette at eye level
Crouch so your eye is level with the bottom of the meniscus for every reading, at the start and the end.
Why: If your eye is above or below the meniscus it lines up with the wrong mark (parallax).
Go drop by drop near the end point
Run acid in quickly until you are about 1 cm³ short of your rough titre. Then turn the tap so it adds one drop at a time, swirling after each drop, until one drop changes the colour for good.
Why: One drop is about 0.05 cm³. Adding the acid in a stream, you only see the change after you have gone past it.
Hazards and control measures
| Hazard | What could happen | Control measure |
|---|---|---|
| Sodium hydroxide solution, 0.100 mol/dm³ (irritant) | Serious eye damage if it splashes into your eyes | Wear goggles throughout, and use a pipette filler: never pipette by mouth. |
| Filling the burette above head height | Acid runs down the burette onto your face and into your eyes | Fill the burette at a low level, below your eyes, with the stand on a stool or the floor. |
| Dilute sulfuric acid (irritant at higher concentrations) | Irritates the skin and eyes | Wipe up spills straight away and wash any acid off your skin with plenty of water. |
| Indicator solutions (harmful if swallowed) | Poisoning if swallowed | Use only a few drops from a dropping bottle and wash your hands afterwards. |
| Glass pipette pushed into the filler | The pipette snaps and cuts your hand | Hold the pipette near its top and push the filler on gently. |
Variables
- Independent: you change
- None: a titration measures one quantity, the titre
- Dependent: you measure
- Titre: the volume of acid needed to neutralise the alkali in cm³
- Control: you keep the same
- Volume of alkali in cm³. 25.0 cm³ from the same pipette every time.
- Concentration of the alkali in mol/dm³. The same 0.100 mol/dm³ bottle throughout.
- Indicator. The same indicator, 5 drops each time.
- End-point colour. Stop at the first permanent change (orange for methyl orange, colourless for phenolphthalein).
Results from a specimen run
| Titration | Initial burette reading in cm³ | Final burette reading in cm³ | Titre in cm³ | Colour at the end |
|---|---|---|---|---|
| Rough | 0.50 | 24.00 | 23.50 | Red |
| 1 | 0.20 | 23.05 | Red | |
| 2 | 0.25 | 23.20 | 22.95 | Orange |
| 3 | 0.10 | 23.10 | 23.00 | Orange |
| Mean (concordant) | blank | blank | 23.00 | blank |
Anomaly spotted: Titration 1: final burette reading, retaken.
Processing the data
- Concordant titres
- Titrations 1, 2 and 3
- Mean titre
- 23.00 cm³
- Moles of sodium hydroxide (Higher tier)
- 0.00250 mol
- Moles of sulfuric acid (Higher tier)
- 0.00125 mol
- Concentration of the acid (Higher tier)
- 0.0543 mol/dm³
- In g/dm³ (Higher tier)
- 5.33 g/dm³
| Quantity | Specimen run | Accepted |
|---|---|---|
| Mean titre | 23.00 cm³ | 22.88 cm³ |
| Acid concentration | 0.0543 mol/dm³ | 0.0546 mol/dm³ |
Across 120 simulated careful groups, concentration of the sulfuric acid (c) came out between 0.0510 and 0.0608 mol/dm³ (median 0.0556). That is the simulation's model, not a promise of what a real class gets.
Common mistakes and the fix
Mistakes to check in this practical, and what to do instead. In a write-up or an exam answer, the mark goes to naming the step and saying what it does to the readings.
Method: Rinse it with water only
The water left inside dilutes the acid, so more of it is needed to neutralise the alkali. Your titres come out high (most at the start, less after each refill) and the acid’s concentration calculates too low (a systematic error).
The fix: Rinse the burette with the acid. Water left inside from washing would dilute the acid you fill it with.
Method: Rinse it with water only
Each 25.0 cm³ portion is slightly diluted, so it holds 0.5–1.5 % less sodium hydroxide. Every titre comes out low and the acid’s concentration calculates too high (a systematic error).
The fix: Rinse the pipette with the alkali. Water left inside would dilute the alkali you measure out.
Method: A little of the sodium hydroxide solution
About 1 cm³ of alkali stays in the flask on top of the 25.0 cm³ from the pipette, so 2–5 % more acid is needed. Titres high, concentration too low (a systematic error).
The fix: Rinse the conical flask. Only the alkali you measure with the pipette should react with the acid.
Rough titration: initial burette reading: Typed 49.5 cm³
The fix: You read the scale upside down. A burette reads 0.00 at the top and the numbers get bigger as you go down.
Rough titration: initial burette reading: Typed 0.35 cm³
The fix: That is where the edges of the meniscus curl up the glass. Read the bottom of the curve, at eye level.
Exam answer: Leaving out: Methyl orange (allow phenolphthalein; not litmus, which changes too gradually)
The fix: Name a single indicator with a sharp colour change, such as methyl orange.
Exam answer: Leaving out: Methyl orange: yellow to red (allow orange or pink); phenolphthalein: pink to colourless. Must be the right way round.
The fix: Acid is added to alkali, so the colour goes from the alkali colour to the acid colour: methyl orange yellow to red.
Worked exam-style questions
Written for Study Hack in the style of exam questions, on the specimen run above. They are not taken from any exam board's papers. Try each one before you open the worked answer.
- Question 1Indicators · Knowledge[2 marks]
A student adds dilute sulfuric acid from a burette to sodium hydroxide solution in a conical flask.
Name a suitable indicator for this titration.
Give the colour change you would see at the end point.
Worked answer and how the marks are given
A full-mark answer
Methyl orange. It changes from yellow to red (orange at the exact end point).
Mark scheme
- Methyl orange (allow phenolphthalein; not litmus, which changes too gradually)
- Methyl orange: yellow to red (allow orange or pink); phenolphthalein: pink to colourless. Must be the right way round. Do not accept “red / orange / pink to / into / then yellow”, “colourless / colorless to / into / then pink”. Only with the mark for: Methyl orange (allow phenolphthalein; not litmus, which changes too gradually).
Where marks are usually lost
- Missing: Methyl orange (allow phenolphthalein; not litmus, which changes too gradually). Name a single indicator with a sharp colour change, such as methyl orange.
- Missing: Methyl orange: yellow to red (allow orange or pink); phenolphthalein: pink to colourless. Must be the right way round.. Acid is added to alkali, so the colour goes from the alkali colour to the acid colour: methyl orange yellow to red.
- Question 2Mean titre · Applying knowledge[2 marks]
Use the results in Table 1.
Calculate the mean titre.
Use only the concordant titres.
Mean titre = cm³
Worked answer and how the marks are given
A full-mark answer
Concordant titres: titrations 1 and 3
Mean = (22.95 + 23.00) ÷ 2
Mean = 22.98 cm³Mark scheme
- Chooses titrations 1 and 3 only (method)
- Mean titre = 22.98 cm³ (allow a correctly calculated mean of an incorrect choice)
- Allow answers from 22.97 to 22.98 cm³ (the marker's tolerance; correct rounding of the student's own working is also accepted).
- Allow the unit written as cm3, ml.
- A correct answer with no working scores full marks.
Where marks are usually lost
- Answer 23.2 cm³ (gains 1 mark). You averaged all four titrations, including the rough one (done quickly to find roughly where the end point is, so it overshoots) and titration 2, which is not concordant. Use only titrations 1 and 3: (22.95 + 23.00) ÷ 2 = 22.98 cm³.
- Answer 23.1 cm³ (gains 1 mark). You used all three accurate titres, but titration 2 is not concordant (it is more than 0.10 cm³ from the others). Mean of the concordant two = 22.98 cm³.
- Answer 46 cm³ (gains 1 mark). That is the total: divide by the number of titres, 2. Mean = 22.98 cm³.
- Question 3Describing the method · Knowledge[6 marks]
A student wants to find the volume of dilute sulfuric acid that exactly neutralises 25.0 cm³ of sodium hydroxide solution.
Describe how the student should do a titration to find this volume.
Worked answer and how the marks are given
A full-mark answer
1. Use a pipette and pipette filler to put 25.0 cm³ of sodium hydroxide solution into a conical flask.
2. Add a few drops of methyl orange indicator and stand the flask on a white tile.
3. Fill a burette with the sulfuric acid and record the initial reading.
4. Add the acid to the flask while swirling, drop by drop near the end point.
5. Stop as soon as the indicator turns from yellow to orange and stays orange when swirled (the end point): red means the end point has been passed.
6. Record the final burette reading. The titre = final reading − initial reading.
7. Repeat until two titres are concordant (within 0.10 cm³) and calculate the mean.Mark scheme
Indicative content:
- Use a pipette (and filler) to measure 25.0 cm³ of sodium hydroxide into a conical flask
- Add a few drops of indicator (methyl orange)
- Fill a burette with the acid and record the initial reading
- Add the acid to the alkali while swirling the flask
- Until the indicator changes colour (permanently): the end point
- Record the final reading; titre = final − initial
- White tile; drop by drop near the end point; read the bottom of the meniscus at eye level; rough titration first
- Repeat until concordant results and calculate a mean
Levels of response Level Marks What the answer does 3 5–6 A coherent method that would give valid results: all the important steps are there, in a sensible order. 2 3–4 A method that might not give valid results: most of the important steps are there, but some are missing or out of order. 1 1–2 A method that would not give valid results: some relevant steps are given, but they are not linked into a workable method. In Study Hack a six-mark answer is not given an automatic level: you tick the points your answer makes, then choose the level that fits it.
- No relevant content: 0 marks.
Evaluate
| Source of error | Type | Effect | Improvement |
|---|---|---|---|
| Judging the end point (the colour change) | Random | You stop a drop or more early or late, a different amount each time, so the titres scatter. | Stand the flask on a white tile, swirl, and add the acid drop by drop near the end point with a sharp indicator (methyl orange or phenolphthalein). |
| Reading the burette (parallax and the 0.05 cm³ resolution) | Random | Each reading can be half a division out, and from above or below the meniscus it lines up with the wrong mark. | Read the bottom of the meniscus with your eye level with it, at the start and at the end. |
| Burette rinsed with water, not the acid | Systematic | The acid is diluted, so the titres are too high and the concentration calculated is too low. | Rinse the burette with a little of the acid before filling it. |
| Pipette rinsed with water, not the alkali | Systematic | Each portion holds slightly less alkali, so the titres are too low and the concentration calculated is too high. | Rinse the pipette with a little of the sodium hydroxide solution before using it. |
| Conical flask rinsed with the alkali | Systematic | Extra alkali in the flask needs extra acid, so the titres are too high. | Rinse the conical flask with distilled water only. |
| Measuring the alkali with a measuring cylinder | Random | The volume of alkali is different each time (±0.5 cm³), so the titres scatter. | Measure the alkali with a 25.0 cm³ volumetric pipette. |
Repeatable. When you repeat the titration with the same solutions and apparatus, your accurate titres are concordant (within 0.10 cm³).
Reproducible. Another group titrating the same acid with different apparatus gets a similar mean titre and concentration.
Other exam boards
How this practical lines up with other GCSE boards' required or core practicals. Codes only; check your own specification for the exact activity.
| Board | Practical | Match | What is different |
|---|---|---|---|
| AQA | Required practical 2 | This page | — |
| Edexcel | 5.9C | Similar, with differences | Hydrochloric acid goes into sodium hydroxide of unknown concentration; this lab uses sulfuric acid. |
Print it
- Blank write-up: the nine-step lab book with the correct table headings, ready to print for a real lesson.
- Specimen lab book: this page's run written up in full, with its exam answers ticked against the mark scheme.
- Printable method card: the practical on one index card. Practise with the cards in Study Hack.
- Practical workbook: a full simulated data set to process on paper, with exam questions and answers.
- Teacher and technician pack: apparatus lists, technician notes and marking points.