A preparation has a theoretical product mass of 7.50 g. After purification and drying, 5.64 g is obtained. Calculate the percentage yield and state why the value alone does not prove purity.
[3 marks]
Total for this question: 3
A-level Chemistry required practicals
Preparing pure organic solids and liquids
AQA 3.3.9.2 · RP10
A-level Chemistry (7405) · Required practical 10 — 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, purify and test a solid organic product and a liquid organic product using two distinct work-up branches.
Apparatus
- Reaction flask, condenser, anti-bumping granules and controlled heat source
- Büchner funnel, filter flask, correctly sized seal and vacuum source
- Hot-filtration apparatus and minimum hot recrystallisation solvent
- Separating funnel, wash solutions and drying agent
- Distillation apparatus with thermometer and receiver
- Balance, melting-point apparatus, capillary tubes and eye protection
Apparatus & techniques (AT)
AT a · measurement
Measures reactant and solid-product masses and obtains a melting range for the solid branch.
AT b · controlled heating
Uses controlled heating for reflux, hot recrystallisation, melting-point work and distillation across the two branches.
AT d · titration/distillation/reflux/qualitative tests/filtration
Uses reflux, filtration and distillation at the appropriate preparation and purification stages.
AT g · recrystallisation or liquid purification
Purifies the solid by recrystallisation and the liquid by separation, drying and distillation.
AT h · melting point
Measures a melting range to assess the identity and purity of the dry organic solid.
AT k · safe handling
Handles flammable, corrosive or harmful organic reagents and heated glassware safely in both branches.
Safety
Hazard
Many organic liquids and recrystallisation solvents are flammable.
Control
Use a water bath, heating mantle or hotplate as appropriate; keep naked flames away and work with ventilation.
Hazard
A closed reflux, distillation or separating-funnel system can build pressure.
Control
Keep heated systems open through the condenser, never seal distillation apparatus, and vent a separating funnel away from people.
Hazard
Hot glassware looks like cold glassware and reduced-pressure vessels may implode if damaged.
Control
Allow glassware to cool before handling, inspect filter flasks and clamp them securely.
Method
- 1Solid branch — synthesis and isolation: carry out the stated reaction under controlled conditions, cool as required and isolate the crude solid.
- 2Solid branch — purification and test: dissolve the crude solid in the minimum hot solvent, hot-filter if insoluble material remains, cool to crystallise, suction-filter, wash with a little cold solvent, dry to constant mass and measure a melting range.
- 3Liquid branch — synthesis and work-up: heat the reaction under reflux, cool, transfer to a separating funnel, separate the layers and wash the organic layer as specified, venting safely.
- 4Liquid branch — purification and test: dry the organic layer, decant or filter it from the drying agent, distil and collect the intended boiling fraction, then measure its boiling range.
- 5For both branches, retain masses or volumes needed for theoretical yield, label every fraction and compare the physical test with reference data.
CPAC focus (editorial)
This is an editorial study focus, not an AQA mapping of fixed CPAC competencies to this practical.
- CPAC 1: Editorial focus: follow each preparation and work-up sequence in the correct order.
- CPAC 2: Editorial focus: choose solvent volume, heating mode and collection fraction deliberately.
- CPAC 3: Editorial focus: control flammability, pressure, corrosive reagents and hot-glass hazards.
- CPAC 4: Editorial focus: record masses, volumes and physical ranges for both products.
- CPAC 5: Editorial focus: calculate yield and separate purity evidence from recovery.
Results & processing
- Calculate theoretical yield from the limiting reagent and percentage yield = actual dry product/theoretical product × 100.
- A pure solid usually melts over a narrow range close to its reference value; a pure liquid gives a narrow boiling range close to reference data.
- Report purity evidence separately from yield: a large mass does not by itself show that the product is pure.
Analysis skills
- Calculate theoretical and percentage yield from the limiting reagent.
- Interpret melting and boiling ranges using both sharpness and agreement with reference data.
- Explain the purpose of reflux, recrystallisation, suction filtration, drying and distillation without confusing their roles.
Uncertainty
Sources
- Balance and volume-reading resolution
- Transfer and volatile-product losses
- Product retained in wash solvent or left wet with solvent
Calculations
- For a mass found by difference, add the absolute uncertainties of the two balance readings.
- Calculate percentage yield only from dry product mass and the stoichiometric theoretical mass.
Interpretation
- Two weighings double the balance contribution to a mass-by-difference uncertainty.
- Low yield can result from purification loss even when the recovered sample is pure; residual solvent can give an apparently high yield but poor purity.
Exam angles
- Label and explain the solid and liquid purification branches separately.
- Correct a suction-filtration, reflux, distillation or melting-point diagram.
- Calculate yield and judge purity from a measured range and reference value.
Where students lose marks
Drawing reduced-pressure filtration with a poor seal or an unsafe connection to the vacuum source.
Fix: Show the Büchner funnel seated with the correct adaptor on a side-arm flask and connect the side arm securely.
- Examiner report: C2-22 · PDF p. 5
Using a large volume of recrystallisation solvent or filtering before the solid has fully dissolved while hot.
Fix: Add the minimum hot solvent needed; hot-filter only to remove insoluble material, then cool to crystallise.
- Examiner report: C2-25 · PDF p. 6
Using a water bath for a temperature it cannot reach, or placing the melting-point sample away from the thermometer bulb.
Fix: Choose a heat source with a suitable temperature range and align the capillary sample with the thermometer bulb before heating slowly near the expected melting point.
- Examiner report: C2-25 · PDF p. 6
Claiming purity only because the boiling point is near a reference value.
Fix: Use both closeness to the reference and a narrow boiling range as the evidence.
- Examiner report: C2-24 · PDF p. 4
Improve the method
- Wash solid crystals with a small volume of cold solvent to remove soluble impurities without dissolving much product.
- Dry the product to constant mass so residual solvent does not inflate yield or broaden the melting range.
- Position the distillation thermometer bulb at the side-arm entrance and collect only the intended boiling fraction.
Source references
- Specification: CSpec 3.3.9.2 · PDF p. 62
Try it — exam-style
Easy
ORIGINAL
Medium
ORIGINAL
A dry solid mass is found by subtracting two balance readings and equals 2.846 g. Each reading has uncertainty ±0.001 g. Calculate the absolute and percentage uncertainty in the product mass.
[3 marks]
Total for this question: 3
Questions are written in the style of past AQA papers — never copied from them.
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