Skip to content
Oscar Song Tutoring
A-level Chemistry required practicals

Volumetric solution and acid–base titration

AQA 3.1.2.5 · RP1

A-level Chemistry (7405) · Required practical 1 — method, techniques, safety, analysis and uncertainty. Includes errors documented in examiner reports.

Board and spec code confirmed against AQA 7405 · registry checked 2026-07-11How this checking works

Prepare a solution of accurately known concentration, then use it in a simple acid–base titration to determine an unknown concentration.

Apparatus

  • Analytical balance, weighing bottle and spatula
  • Beaker, glass rod, funnel and wash bottle of deionised water
  • Volumetric flask with stopper
  • Volumetric pipette and pipette filler
  • Burette, stand, white tile and conical flask
  • Suitable acid–base indicator

Apparatus & techniques (AT)

AT a · measurement

Record the mass and all burette readings to the precision supported by the balance and scale.

AT d · titration/distillation/reflux/qualitative tests/filtration

Deliver titrant from a conditioned burette into a measured aliquot and judge the endpoint consistently.

AT e · volumetric-flask technique

Transfer the dissolved solid quantitatively, make the solution to the calibration mark and mix it thoroughly.

AT f · acid–base indicators

Choose an indicator whose transition range lies within the steep pH change around the equivalence point.

AT k · safe handling

Handle acids, alkalis and glassware safely, using a pipette filler rather than pipetting by mouth.

Safety

Hazard

Acid and alkali solutions can irritate or burn skin and eyes.

Control

Wear eye protection, use a pipette filler, clean splashes promptly and rinse exposed skin with plenty of water.

Hazard

A clamped burette or volumetric glassware can break if knocked or forced.

Control

Clamp the burette vertically away from the bench edge and handle stoppers and glassware without force.

Method

  1. 1Weigh a suitable mass of the solid into a clean weighing vessel and record the mass.
  2. 2Transfer the solid to a beaker, rinse the weighing vessel into the beaker and dissolve the solid in a small volume of deionised water, using a glass rod to mix.
  3. 3Transfer the solution through a funnel into a volumetric flask; rinse the beaker, rod and funnel into the flask so the transfer is quantitative.
  4. 4Add water to near the calibration mark, then use a dropping pipette until the bottom of the meniscus is on the mark at eye level. Stopper and invert repeatedly to mix.
  5. 5Condition the pipette with the solution it will deliver, transfer one aliquot to a conical flask and add only a few drops of a suitable indicator.
  6. 6Condition and fill the burette with the other solution, remove the funnel, check the jet for air bubbles and record the initial reading at eye level.
  7. 7Run a rough titration, swirling the flask, then repeat with dropwise addition near the endpoint until concordant titres are obtained.

CPAC focus (editorial)

This is an editorial study focus, not an AQA mapping of fixed CPAC competencies to this practical.

  • CPAC 2: Editorial focus: choose calibrated glassware and an indicator that make the intended concentration measurement valid.
  • CPAC 4: Editorial focus: record initial and final burette readings, the endpoint and sufficient repeat titres without overwriting raw data.
  • CPAC 5: Editorial focus: justify the concordant mean and carry the titre through a balanced-equation concentration calculation.

Variables

Independent

Volume of titrant delivered from the burette

Dependent

Indicator endpoint and the concentration calculated from the endpoint titre

Control

  • Aliquot volume and identities of both solutions
  • Indicator type and number of drops
  • Temperature and endpoint judgement

Results & processing

  • Tabulate initial and final burette readings and calculate each titre by subtraction; keep the rough result visible but exclude it from the concordant mean.
  • Select titres that agree within the stated concordance criterion and calculate their mean without choosing values merely because they are close to an expected answer.
  • Use the balanced equation to convert titrant moles into analyte moles, then divide by the aliquot volume in dm3 to obtain concentration.

Analysis skills

  • Calculate moles in the prepared solution from mass and molar mass, then concentration from moles divided by volumetric-flask volume.
  • Treat each titre as final burette reading minus initial burette reading and justify which repeat values form the concordant mean.
  • Apply the acid–base stoichiometric ratio before calculating the unknown concentration.

Uncertainty

Sources

  • Balance resolution, volumetric-flask tolerance, pipette tolerance and two burette readings all contribute.
  • Meniscus alignment, a remaining burette-jet bubble and subjective endpoint judgement can bias the result.

Calculations

  • For a burette with +/-0.05 cm3 reading uncertainty, a titre uses two readings and therefore has +/-0.10 cm3 absolute uncertainty.
  • Calculate each percentage uncertainty against the volume measured by that apparatus; add percentage uncertainties for multiplied or divided quantities when estimating a maximum combined uncertainty.

Interpretation

  • A smaller titre gives a larger percentage burette uncertainty even though the absolute reading uncertainty is unchanged.
  • Concordant titres show repeatability, but they do not reveal a shared systematic error such as a wrongly made standard solution.

Exam angles

  • Calculate the solid mass needed for a stated solution volume and concentration, including purity where supplied.
  • Explain quantitative transfer, correct rinsing and why adding deionised water to the conical flask does not change the reacting moles.
  • Select an indicator, identify concordant titres and calculate an unfamiliar acid–base stoichiometry.
  • Calculate absolute and percentage apparatus uncertainty and explain why a smaller titre is less precise.

Where students lose marks

Reading a meniscus from above or below eye level, or using the wrong part of the meniscus.

Fix: Read the specified meniscus at eye level against a clear background to avoid parallax.

  • Examiner report: C3-21 · PDF p. 3

Adding the pipette and burette absolute uncertainties, then dividing by an unrelated total volume.

Fix: Convert each apparatus uncertainty using the volume that apparatus measured before combining percentage contributions.

  • Examiner report: C1-23 · PDF p. 5

Washing the conical flask with analyte solution or rinsing the burette with water immediately before filling.

Fix: Rinse calibrated delivery glassware with the solution it contains; rinse the conical flask only with deionised water.

Improve the method

  • Use a rough titration to locate the endpoint, then add titrant dropwise while washing flask-wall splashes down with deionised water.
  • Obtain at least two concordant titres and use only the justified concordant set for the mean.
  • Choose a titre large enough to limit percentage burette uncertainty while keeping all volumes within the apparatus range.

Source references

  • Specification: CSpec 3.1.2.5 · PDF p. 17
  • Specification: CSpec 3.1.2.5 · PDF p. 18

Try it — exam-style

Medium
ORIGINAL

Calculate the mass of anhydrous Na2CO3, Mr = 106.0, needed to make 250.0 cm3 of 0.1000 mol dm−3 solution.

[2 marks]

Total for this question: 2

Hard
ORIGINAL

25.00 cm3 of NaOH is titrated with 0.0800 mol dm−3 H2SO4. A rough titre is 20.80 cm3 and accurate titres are 20.40, 20.35 and 20.30 cm3. Use H2SO4 + 2NaOH → Na2SO4 + 2H2O to calculate the NaOH concentration.

[5 marks]

Total for this question: 5

Questions are written in the style of past AQA papers — never copied from them.

Drill it properly

A-level Chemistry specification coverage

Drill amount of substance, equilibria and the rest of AQA 7405 by specification point.

Stuck on volumetric solution and acid–base titration?

Titration marks come from disciplined technique and disciplined working — I drill both. Free intro call, then a free first lesson.

Book a free intro call