Neutralisation

1 Plan and risk assessment

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.

2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l)

Prediction

2 Apparatus

• ✓Safety goggles Protect your eyes from the alkali and the acid.
• ✓50 cm³ burette (resolution 0.05 cm³) Adds the acid and measures how much was added.
• ✓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 (resolution ±0.06 cm³) Measures exactly 25.0 cm³ of alkali.
• ✓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.

3 Method: your choices

• Which version are you doing?: AQA required practical 2: sulfuric acid of unknown concentrationSulfuric acid in the burette, 25.0 cm³ of 0.100 mol/dm³ sodium hydroxide in the flask; find the acid’s concentration (2 : 1).
• Rinse the burette with the acid: DoneWater left inside from washing would dilute the acid you fill it with.
• Rinse the pipette with the alkali: DoneWater left inside would dilute the alkali you measure out.
• Rinse the conical flask: Distilled water (it can stay wet)No effect: water in the flask does not change the number of moles of alkali the pipette puts in.
• Fill the burette at a low levelSafety: DonePouring acid above head height can run it down the burette onto your face and into your eyes.
• Fill the jet below the tap: DoneAn 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: DoneAcid left in the funnel can drip into the burette after you have read it.
• Choose how to measure the alkali: 25.0 cm³ volumetric pipette and fillerDelivers 25.0 cm³ to within about 0.06 cm³ every time: fill to the line at eye level, let it drain, touch the tip on the flask, do not blow out.
• Choose the indicator: Methyl orangeYellow in the alkali; one drop of acid at the end point turns it orange, then red. A sharp change.
• Stand the flask on a white tile: DoneAgainst white, the first permanent colour change is much easier to see.
• Do a rough titration first: DoneThen in the accurate titrations you can add acid quickly to about 1 cm³ before the end point, and go drop by drop from there.
• Read the burette at eye level: DoneIf your eye is above or below the meniscus it lines up with the wrong mark (parallax).
• Go drop by drop near the end point: DoneOne drop is about 0.05 cm³. Adding the acid in a stream, you only see the change after you have gone past it.

• 0.0 Struck through: the anomaly, retaken; the new reading is beside it.

Anomaly: Titration 1: final burette reading, retaken. Likely cause: a one-off slip in that titration: overshot the end point, or misread a reading.

5 Graph

This practical has no graph: the results are in the table above.

6 Analysis

1. Concordant titres

   Which titres are concordant (within 0.10 cm³ of each other)? Use only those for the mean.

   Titrations 1, 2 and 3 Right first time

2. Mean titre

   mean = sum of the concordant titres ÷ number of them

   Calculate the mean of your concordant titres, to 2 decimal places like the burette readings.

   23.00 cm³ Right first time

3. Moles of sodium hydroxide (Higher tier)

   moles = concentration × volume in dm³

   Calculate the number of moles of sodium hydroxide in 25.0 cm³ of 0.100 mol/dm³ solution.

   0.00250 mol Right first time

4. Moles of sulfuric acid (Higher tier)

   2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l)

   Use the equation to work out the moles of sulfuric acid that reacted.

   0.00125 mol Right first time

5. Concentration of the acid (Higher tier)

   concentration = moles ÷ volume in dm³

   Calculate the concentration of the sulfuric acid in mol/dm³. Give your answer to 3 significant figures.

   0.0543 mol/dm³ Right first time

6. In g/dm³ (Higher tier)

   mass concentration = concentration × Mr (Mr of H₂SO₄ = 98)

   Your acid is 0.0543 mol/dm³. Give its concentration in g/dm³.

   5.33 g/dm³ Right first time

7 Conclusion and evaluation

Conclusion

Sources of error

Improvement

Repeatable or reproducible

9 Exam questions on your data

On your own readings, self-marked against the mark scheme, with error carried forward where it applies. Total 30/30.

1. 01.1[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.

   Your answer

   Methyl orange. It changes from yellow to red, orange at the exact end point.

   2/2 self-marked

   • 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.

   Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

   Model answer: Methyl orange. It changes from yellow to red (orange at the exact end point).

2. 01.2[2 marks]

   Explain why the sodium hydroxide solution is measured with a pipette, but the acid is added from a burette.

   Your answer

   A pipette measures a fixed volume of 25.0 cm³ accurately. The burette measures a variable volume and can add the acid drop by drop.

   2/2 self-marked

   • A pipette measures a fixed volume (accurately)
   • A burette measures a variable volume (allow can add drop by drop)

   Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

   Model answer: A pipette measures one fixed volume of alkali (25.0 cm³) accurately. A burette measures a variable volume: it shows how much acid was needed, and can add it drop by drop.

3. 01.3[2 marks]

   Give two things the student should do to find the end point accurately.

   Your answer

   Swirl the flask while adding the acid, and add the acid drop by drop near the end point.

   2/2 self-marked

   • Swirl the flask (as the acid is added)
   • Stand the flask on a white tile
   • Add the acid drop by drop (dropwise) near the end point
   • Do a rough titration first (to find the approximate end point)
   • Repeat and calculate a mean (of concordant titres)
   • Read the bottom of the meniscus at eye level

   Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

   Model answer: Any two from: swirl the flask; stand it on a white tile; add the acid drop by drop near the end point; do a rough titration first; repeat to get concordant titres and calculate a mean.

4. 01.4[1 mark]

   The method says the burette should be filled with its top below the student’s eye level.

   Suggest why.

   Your answer

   So that if acid spills it does not run down onto your face and into your eyes.

   1/1 self-marked

   • Acid could spill or splash into the eyes or onto the face (allow skin)

   Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

   Model answer: So that acid cannot spill or run down onto the face and into the eyes while the burette is being filled.

5. 02.1[1 mark]

   Another student titrated the same acid against 25.0 cm³ portions of the sodium hydroxide solution.

   Table 1 shows the student’s results.

   Which accurate titration is not concordant with the others?

   Select one row.

   Table 1

   Titration	Initial burette reading in cm³	Final burette reading in cm³	Titre in cm³
   Rough	0.00	23.60	23.60
   1	0.60	23.55	22.95
   2	0.20	23.60	23.40
   3	1.00	24.00	23.00

   Your answer

   titration 2

   1/1

   • Titration 2: 23.40 cm³ is more than 0.10 cm³ from the other two

   Full marks. Your answer meets all the points for this question. Titration 2 (23.40 cm³) is not within 0.10 cm³ of the other accurate titres, which agree. The rough titration is not an accurate titration.

   Model answer: Titration 2. Its titre is more than 0.10 cm³ from the other two accurate titres, which agree with each other.

6. 02.2[2 marks]

   Use the results in Table 1.

   Calculate the mean titre.

   Use only the concordant titres.

   Your answer

   22.98

   Working: Concordant titres: titrations 1 and 3 Mean = (22.95 + 23.00) ÷ 2 Mean = 22.98 cm³

   2/2

   • Chooses titrations 1 and 3 only
   • Mean titre = 22.98 cm³ (allow a correctly calculated mean of an incorrect choice)

   Full marks. That is exactly what the mark scheme is looking for.

   Model answer: Concordant titres: titrations 1 and 3 Mean = (22.95 + 23.00) ÷ 2 Mean = 22.98 cm³

7. 02.3[1 mark]

   Give one reason why the rough titration is not used to calculate the mean titre.

   Your answer

   It was done quickly just to find the approximate end point, so it overshoots and is too high.

   1/1 self-marked

   • It is only approximate (done quickly to find the approximate end point), so it overshoots / is too high (allow it is the rough value)

   Self-marked: 1 of 1 mark. This is your own judgement against the mark scheme.

   Model answer: It was done quickly to find the approximate end point, so it overshot: it is higher than the accurate titres.

8. 03.1[5 marks]

   This question is for Higher tier.

   The equation for the reaction is:

   2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l)

   • Volume of sodium hydroxide solution: 25.0 cm³
   • Concentration of sodium hydroxide solution: 0.100 mol/dm³
   • Mean titre of sulfuric acid: 23.00 cm³

   Calculate the concentration of the sulfuric acid in mol/dm³.

   Give your answer to 3 significant figures.

   Your answer

   0.0543

   Working: Moles of NaOH = 0.100 × 25.0 ÷ 1000 = 0.00250 mol Moles of H₂SO₄ = 0.00250 ÷ 2 = 0.00125 mol Volume of acid = 23.00 ÷ 1000 = 0.023 dm3 Concentration = 0.00125 ÷ 0.023 = 0.0543 mol/dm3

   5/5

   • Moles of NaOH = 0.100 × 25.0 ÷ 1000 = 0.00250
   • Moles of H₂SO₄ = 0.00250 ÷ 2 = 0.00125 (allow their moles of NaOH ÷ 2)
   • Concentration = 0.00125 ÷ (23.00 ÷ 1000) (allow inverted expression; allow their moles)
   • = 0.0543478 (allow ECF)
   • = 0.0543 mol/dm³ (3 significant figures)

   Full marks. That is exactly what the mark scheme is looking for.

   Model answer: Moles of NaOH = 0.100 × 25.0 ÷ 1000 = 0.00250 mol Moles of H₂SO₄ = 0.00250 ÷ 2 = 0.00125 mol Volume of acid = 23.00 ÷ 1000 = 0.023 dm³ Concentration = 0.00125 ÷ 0.023 = 0.0543 mol/dm³

9. 03.2[2 marks]

   This question is for Higher tier.

   Relative formula mass (Mr): H₂SO₄ = 98

   Calculate the concentration of the sulfuric acid in g/dm³.

   Use your answer to the previous question.

   Your answer

   5.33

   Working: Concentration in g/dm³ = 0.0543 × 98 = 5.33 g/dm³

   2/2

   • 0.0543 × 98 (allow their answer to the previous question × 98)
   • = 5.33 g/dm³ (allow ECF)

   Full marks. Your answer meets all the points for this question.

   Model answer: Concentration in g/dm³ = 0.0543 × 98 = 5.33 g/dm³

10. 03.3[2 marks]

    This question is for Higher tier.

    A technician made 100 cm³ of 0.100 mol/dm³ sodium hydroxide solution by dissolving solid sodium hydroxide in water.

    Relative formula mass (Mr): NaOH = 40

    Calculate the mass of sodium hydroxide the technician dissolved.

    Your answer

    0.40

    Working: Moles of NaOH = 0.100 × 100 ÷ 1000 = 0.01 mol Mass = 0.01 × 40 = 0.40 g

    2/2

    • Moles = 0.100 × 100 ÷ 1000 = 0.01
    • Mass = 0.40 g (allow their moles × 40)

    Full marks. That is exactly what the mark scheme is looking for.

    Model answer: Moles of NaOH = 0.100 × 100 ÷ 1000 = 0.01 mol Mass = 0.01 × 40 = 0.40 g

11. 04.1[2 marks]

    Another student washed the burette with water, but did not rinse it with the acid before filling it.

    Explain how this affects the concentration of acid the student calculates.

    Your answer

    Water left in the burette dilutes the acid, so a larger titre is needed and the calculated concentration is lower than the real value.

    2/2 self-marked

    • The water left in the burette dilutes the acid
    • So the titre is larger and the calculated concentration is too low

    Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

    Model answer: The water left in the burette dilutes the acid, so more acid is needed to neutralise the alkali. The titre is larger, so the calculated concentration is too low.

12. 04.2[2 marks]

    A student’s conical flask was still wet with distilled water when they pipetted 25.0 cm³ of sodium hydroxide solution into it.

    Explain why this does not affect the titre.

    Your answer

    The number of moles of sodium hydroxide is the same. Water does not react with the acid, so the volume of acid needed stays the same.

    2/2 self-marked

    • The number of moles of sodium hydroxide in the flask is the same
    • Water does not react with the acid (only the moles of alkali decide how much acid is needed)

    Self-marked: 2 of 2 marks. This is your own judgement against the mark scheme.

    Model answer: The pipette still puts the same number of moles of sodium hydroxide into the flask. Water does not react with the acid, so the same volume of acid is needed to neutralise it.

13. 05[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.

    Your 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.

    6/6 self-marked

    • 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

    Self-marked: Level 3, 6 of 6 marks. You ticked 8 of 8 indicative points. This is your own judgement against the mark scheme.

    Model 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.

Unseen data: the same skills, new apparatus

1. 01[2 marks]

   A student performs a separate simulated titration. The burette readings are shown below.

   Burette readings

   Initial reading in cm³	Final reading in cm³
   0.45	23.10

   Calculate the volume of acid delivered.

   Your answer

   22.65

   Working: 23.10 - 0.45

   2/2

   • Subtracts the initial reading from the final reading.
   • 22.65 cm³

   Full marks. Clear and precise.

   Model answer: 23.10 − 0.45 = 22.65 cm³

2. 02[2 marks]

   These titres are from another simulated investigation. Treat accurate titres within 0.10 cm³ of one another as concordant. Use the largest concordant set; leave out the rough result.

   Delivered volumes

   Titration	Titre in cm³
   Rough	25.80
   1	25.15
   2	24.40
   3	24.45

   Calculate the mean of the concordant accurate titres.

   Your answer

   24.43

   Working: 24.40 + 24.45 = 48.85 48.85 / 2

   2/2

   • Adds the concordant titres (2, 3): 48.85 cm³.
   • Mean = 24.43 cm³.

   Full marks. Your answer meets all the points for this question.

   Model answer: Use accurate titrations 2 and 3. Their sum is 48.85 cm³. Mean = 48.85 ÷ 2 = 24.43 cm³.

3. 03[1 mark]

   A student obtains these concordant simulated titres: 24.40 cm³, 24.45 cm³. Before every titration, the pipette was rinsed only with water, leaving water inside before the sodium hydroxide solution was added.

   Which conclusion is justified? Tick (✓) one box.

   • ✓ The close titres support repeatability, but water in the pipette can dilute the measured solution and produce a systematic error. (your choice)
   • Concordant titres prove the method has no systematic error.
   • Taking a mean removes any systematic error caused by the water.
   • Water in the pipette has the same effect as water added to the flask after the measured solution is delivered: neither can change the amount of sodium hydroxide delivered.

   Your answer

   The close titres support repeatability, but water in the pipette can dilute the measured solution and produce a systematic error.

   1/1

   • The close titres support repeatability, but water in the pipette can dilute the measured solution and produce a systematic error.

   Full marks. Well done.

   Model answer: Close titres show repeatability. Rinsing the pipette with the solution it will deliver avoids diluting the measured portion. Repeats and a mean do not remove this systematic method error.