Laboratory Practice, Data Analysis and Experimental Technique
This domain tests whether you can choose apparatus, carry out standard procedures correctly, record data to an appropriate precision, and interpret the numbers critically.
Apparatus and measurement
Use a burette for variable accurate volumes, a pipette for a fixed aliquot, a volumetric flask for making standard solutions, and a measuring cylinder only where approximate volumes are acceptable. Read the bottom of the meniscus at eye level to avoid parallax. A burette is read to ±0.05 cm³ and each titre uses two readings, so the uncertainty is ±0.10 cm³; a 25.00 cm³ pipette is typically ±0.06 cm³. Electronic balances are read to 0.01 g or 0.001 g depending on the model.
Standard procedures
Titration: rinse the burette with the titrant, the pipette with the analyte, remove the funnel, run a rough titre, then titrate to a permanent faint colour, reading to the bottom of the meniscus. Gravimetric work: heat to constant mass, cool in a desiccator, and repeat until successive masses agree. Recrystallisation: dissolve in the minimum hot solvent, filter hot to remove insoluble impurities, cool, filter, wash with cold solvent, dry.
Errors and data handling
Random errors scatter and are reduced by repetition; systematic errors shift every result in the same direction (for example a mis-calibrated balance). Percentage error = (uncertainty/measurement) × 100. In graphs, plot the independent variable on the x-axis, draw a line of best fit rather than joining points, and take gradients over the largest possible interval. Report results to the precision of the least precise measurement.
Worked example
A titre is recorded as 23.45 cm³ from readings 0.00 and 23.45. Each reading has uncertainty ±0.05 cm³, so the titre has ±0.10 cm³. Percentage error = 0.10/23.45 × 100 = 0.43%.
Traps that cost marks
- Leaving the funnel in the burette, which adds uncontrolled titrant.
- Rinsing the pipette with water instead of the solution being measured, diluting it.
- Recording a titre to 0.001 cm³ when the burette cannot deliver that precision.
- Overshooting the end-point and then ignoring the resulting systematic error.
- Adding indicator in large amounts, which itself consumes titrant.