Laboratory Practicals and Experimental Techniques
Laboratory Practicals and Experimental Techniques
This domain tests safe and accurate experimental work: separation and purification, preparation of solutions, titration technique, calorimetry, yield calculations and identification of errors. Questions often ask why a step is done, not just what is done.
Core rules
- Filtration separates an insoluble solid from a liquid; distillation separates liquids by boiling point; recrystallisation purifies a solid; a separating funnel separates immiscible liquids.
- Use a pipette to deliver a fixed volume and a burette for variable volumes. Read the bottom of the meniscus at eye level.
- In recrystallisation, dissolve the solid in the minimum hot solvent, filter hot if needed, cool to crystallise, filter, wash with cold solvent and dry.
- Percentage yield = actual mass/theoretical mass × 100. Melting point comparison checks purity; impurities lower and broaden the melting range.
- In calorimetry, use a insulated container, measure temperature change accurately and account for heat loss.
Worked example
A student prepares aspirin and obtains 3.20 g from a theoretical yield of 4.00 g. Percentage yield = 3.20/4.00 × 100 = 80.0%. To improve accuracy, the product should be washed with cold solvent to remove soluble impurities without dissolving much product, then dried to constant mass before weighing.
Traps
Do not rinse the pipette with water before measuring acid; rinse it with the solution. Do not weigh wet crystals. Do not average a rough titration with accurate titres. A percentage error calculation must use the measured value and accepted value, not the yield formula.