Practical and Experimental Skills, Data Handling and Laboratory Safety
This domain tests accurate measurement, correct technique, data processing and safe laboratory practice. STPM expects candidates to identify errors and choose appropriate apparatus.
Measurement and errors
Accuracy is closeness to the true value; precision is closeness of repeated measurements. Systematic errors shift all results in one direction; random errors scatter results. Record burette readings to 0.05 cm3 and use the meniscus at eye level. Significant figures should reflect the precision of the apparatus.
Techniques and data handling
Titration requires rinsing the burette with the titrant and the pipette with the solution being measured. Repeat titrations until concordant. Recrystallisation purifies solids; filtration separates insoluble solids; distillation separates liquids by boiling point. Plot graphs with labelled axes, suitable scales and a line of best fit.
Safety
Read hazard labels and MSDS. Wear goggles. Use a fume cupboard for volatile or toxic substances. Add concentrated acid to water, never water to concentrated acid. Dispose of waste in the correct container. Never taste chemicals or point a test tube towards anyone.
Worked example
A burette has an uncertainty of ±0.05 cm3 per reading. A titre uses an initial and a final reading, so the total uncertainty is ±0.10 cm3. For a titre of 25.00 cm3, percentage uncertainty = (0.10/25.00) × 100 = 0.40%.
Traps that cost marks
Avoid parallax error when reading the meniscus. Do not forget to rinse apparatus with the correct solution. Choose an indicator whose colour change matches the titration. Do not quote more significant figures than the apparatus allows. Never add water to concentrated acid.